CA1101022A - Load sensing control valve - Google Patents

Abstract

LOAD SENSING CONTROL VALVE

ABSTRACT OF THE DISCLOSURE

A valve for use in vehicle hydraulic brake systems for the purpose of limiting the rear wheel brake pressure relative to the front wheel brake pressure. in order to reduce the potential for premature rear wheel lock,. The valve includes a proportioning piston which operates to limit the fluid pressure delivered to the rear wheel brakes and an inertia member which senses changes in vehicle loading. When a vehicle is fully loaded, the inertia member remains in a position where it will disable the proportioning piston in its open position and, consequently, The rear wheel brake pressure will be equal to the front wheel brake pressure. When the same vehicle is unloaded, the inertia member moves in response to deceleration to a position where it will not: disable the proportioning piston in its open posi-tion so that the rear wheel brake pressure will be reduced relative to the front wheel brake pressure.
The valve may also include a valve assembly which by-passes the proportioning piston to increase the rear wheel brake pressure subsequent to a period when the rear wheel brake pressure is limited.

Description

2~:
13~ROUND OF l'llE INVENTTON
It is known that changes in vehicle loading .
cause changes in ~raking capability~ Fox lnstc~nce, when a vehicle is iII the fully loaded conditiong the rear wheels will have nearly the same braking capability as .
the front wheels. Howevex, when the same vehicle is in the unladen condition, the rear wheels may have substan-tially less braking capabili~y than the ~ront ~heels.
Thus, the potent~al for premature rear wheel lock may be much greater when s~opping ~he unlade~ vehicle than when:
stopping the fully loaded vehlcle. This is particularly S apparent in truck-type vehicles and subcompac~ passengex vehicles. . .
Vehicle hydraulic brake systems have included .:
various types of pressure control valvles which r,educe the potential ~or premature rear wheel lock during brake ~ :
O appl~cations. ConventionaL pressure control valves limi~
the xear wheel brake pressure xelati~e to the ront wheel brake pressure in response to a predet~ermined level o . .
master cylinder pxessureO One such type o~ pressure con-'cxol valve is shown in UO SO Patent No. 3,42339360 In -S the unladen vehicle condi~ion~ this type o valve is ...
advantageous sinca it pxevents premature xeax wheel lock l;
yet allows the bxake system to pxoduce the total brake torque that is needed or stopping the unladen vehicle in a prede~ermi~ed distance~ Howe~er, this type o~ val~e i ~s not entixely desirable in the ully loaded vehicle ¦
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. r condition when there is a substantial difference between .
the ~ully loaded weight and the unladen weight o~ a vehicle sueh as in a truck. The principle reason that this type- t~of ~alve may be undesirable in the fully loaded conditio~
is that the brake system may not be a~le to produce the total brake toxque that is needed for stopping the fully loaded vehicle ~n the same distanee as t:he unladen vehicle.
In an attempt to overcome this problem~ a similar type of pressure control valve has include~ a.'~lend-backl' ~ype bypass feature which steadily increases the rear brake pressure subse~uent to a predetermined period when . .
the rear brake pressure is limited. The brake sy~tem is thus able to provide the total brake torque tha~ is needed .
or stopplng the ~ully loaded veh:Lcle i~ the same d~stance as the unladen vehicleO Thls typle of valve has a ~isad-vantage in that the wear rate o~ the front bra~es is ln-creased when the vehicle is fully loaded9 Another dlsad- .
vantage of this type of valve is that the pote~tial fo~ ¦
$ro~t wheel lock is increased when ~toppi~g ~he fully loaded vehicle on wet or icy surfaces.
~e~e ~ndesirable f ea'cures and disadvantages of .
the pr~or ~rt pressure corltrol vals~Ps are more evident on truck type vehicles and subcompaot passenger veh~cles : . tha~ on intermediate and full-size passenger vehicles~
~ YY~IYL~YIY_YSYY~119Y .
T~e present inventlon improves the brak~ng cap~
a~ility o~ the unladen vehicle while maintaining the designed braking capability of the fully loaded vehiclQ5 The pxeerred embcdiment o~ the present inventîon includes - 30 means which automatically senses vehicle loading conditions ! ¢

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so that the reax wheel brake pressure is limited ~7hen a vehicle is unladen but is not limited when the same .
vehicle is fully loadedO
The present invention provides a control valvè
whlch ~ncludes an inlet ~or receiving fluid pressure from a master cylinder, an outlet for delivering fluid pressure .
to a wheel brake, and valving me~ns or transmitting fluid pressure at th~ inlet to the outlet and for limiting the 1uid pressure transmitted to the outlet relative to the .
~luid pressure at the inlet. The present invention also provldes i~hibiting means for preventing the valving means .
from limi~ing the fluid pressure tra~smitted from the ~
inlet to the outlet in order to maintain the outlet ~:
pressure equal to the inlet pressure for the ull range -. of ma~ter cylinder pressuxe. With the present inventio~
installed on a vehi~le in the preerred manner for con~
trolling ~lu~ flow between the master cylinder and the .
rear ~heel brakes, the rear wheel brake pressure will be . . regulated i~ accordance with the vehlcle loading condition~
~n particular when the vehicle is ully loaded, the in-hibiting means will be utilized and the valving means ~
will b~ prevented from limiting the fluid pressu~e delivered ~:
to the rear whe~l brakes during a braking applica~ion so ~
. that the rear wheel brake pxessuxe will ~e equal to the :
master cylinde~ pressure for all levels o~ master cylinder pressureO When the vehicle is Imloaded ox lightly loaded~
the inhibiting means will not be utiliæed ~nd the valving means will limit the fluid pressu~e delivered to th~ rear wheel brakes during a braking application so ~hat the , rear wheel brake pressure will be xeduced relative to the master cylinder pressureO `
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FIGo 1 is a schematic vie~ o:f a brake system in- ~-corporating a corltrol valve according to the preseIIt ~nvent ~on; : `
FIGo 2 i.s an enlarged vertlcal sectlonal ~iew of ~.
a first e~bodiment of the control valve of the present . . .
- inveIltion;
~IG. 2a is a view of two of the parts of the con- .
~rol valve illustrated in FIG. 2;
FIG. 3 is aII enlarged view~ partly in section and partly in elevation9 of one of the part of the control `
valve illustrated in. FIG. 2; : ;
FIG. 4 i~ an exld view o~ a portion of the valve element illustrated in FIG. 3; ~
FIG,. 5 i9 a sectional vi~ of the valve element - .
- of FIG O 3 taken along lines 5- 5 ther eof; ¦ ` :
FIG. 6 is a ~ectional view o a portion o~ the valve elemellt o~ FIG. 3 taken alonLg lines 6-6 thexeo~
~IG., 7 is a sectional v~ew o~ a portion of the control valve o~ FIG. 2 taken along lines 7-7 thereof, -FEGo 8 is a sectional view of a portiorl of the control valve of FIG~ 2 taken ~long ll~e~ 8-8 thereof;
FIG. 9 ~ s a graph comparing inlet pre~sa~re and -outlet pressure for th2 control valves o the present --_~
ln~ention; : ' FIG. 10 is an enlarged vertical sectional vlew ~ -~ ~.
o~ a second embodlment of the oontrol Yalve of the pre-~cnt ~ve~tion;
FIG. 11 is an enlarged vertical sectional view ~ ~
of a third emSodiment of the control val~re o~ the pxe- ~.7.
sent inven~ion; . ~-~
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r FIG. 12 is a schematic view of a brake system .
incorporating a fouxth embodiment of the control valve .
of the pxesent invention shown in vertical section;
FXG. 12a is a view of two o the parts of ~he COlltXGl valve illustrated in ~IG. 12; and .
FIG. 13 is a sectional view o a portion o ~
tha control valve o:E FIGo 12 taken along lines 13-13 ::-thereo~ .

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. r ~ . , , Ref rring now to the drawings~ a portion of a bxake system 10 i~ generally illustrated in FIG. 1 and ~cludes standard components such as a ~aster cyllnder :~
11 operated by a br~lce pedal 12. Brake fluid ~n the master cylinder 11 is del~Yered to the frant brake wheeL
cylinders 13a, 13b via conduit 14 alld branch conduit~ .-~4a3 14b. Brake ~luid i~ also delivered to ~he rear ~rake ~heel c~linders 16aa 16b via conduit 179 control valve 18, conduit l9 a~d branch coIldui~s l9a, l9b~ . ~ ::
The novel control valve 18, as show~ L detail - ~
FIGo 2~ include~ a housing 20 preferably foxmed o a - ~ . -. ~ suitable metal. The housi~g 20 i:~cludes an inlet bos~ ~ . ~:
21 having a bore 22 with an outer threaded porti~n 2~. ~.
15 A fitting 24 having a hexagonal h~ead a~d a threaded - .
portion i~ xeceived irl the threadled bo~3 portion 23, A ~:
seal 25 i8 disposed be~een the housing ~0 and the fi~ting ¦
24. The ~it'cing 24 includes aI~ let open~g 26 for ~ .:.
receiv~g conduit 170 A fluld pas~age 27 co~ects the ~ ~-inlet s~pen~ng 26 and the bore 220 The housinig 20 urther ~cludes al~ outlet boss 28 having arl ou~let oper~ing 29 - 1~
for recei~ing conduit 19. ~ . : ~ :.
- The housing 20 al~o i~ac1udes a l~:entral cavitg 30 ~ which is connected to the bore 22 by the 1uid pas~age 31. The cavity 30 is also connec:ted to the outlet opeIIing 29 by a 1uid passage 320 The cavity 30 includes a . threaded portio~ 33 a~ its upper end~ A cap 34 hav~ng a threaded portîon 36 is received in the ~hreaded portion ¦
33 o the cavit~ 30. A seal 37 ~s disposed bet:ween the housing 20 and the cap 34 to prever~t leakage of fluld9 .
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'""': ~ f The cap 34 lncludes a central blind bore 38 open toward .
the cavity 30 and disposed coaxially ther~with. Cap 34 also includes a countexbore 39 coaxial with the bore 38O .
Counterbore 39 defines a peripheral wall 40 ha~ring a .
bottom annular s-lrface 41P Cap 34 also has fluid pasæ-ages 42 extending through the peripheral wall 40 and spaced 180 apart.
~Housing 20 fuxther include~ a lower hollo~ por- :-tion 43 defining a chamber 44, preferably cylindrieal, with a closed elld 46 and an ope~ end 47O A weighted h~membex 48, preferably spherieal, is posit oned irl the chamber 44., The weighted member 48 is~ormed of a rigid7 ~
~on-resilient material sueh as s~eelO In this particular embodimeIIt of the invention, the weighted meml~er 48 rolls or slides in xesponse to vehiele decelaration along the `
longitudinal axis 49 o the char~ber 44~ A plug 50 is :
disposed in the open end 47 o~ thle chamber 44 and is .
~ecured therein by a retainer rin~g 51. The rolling or ~liding mavement of the weighted me~er 48 to the righ~ :
. 2t) in F1G. 2 is llmited by the i.nn~r end 52 of the plug 50 and the rolling or sliding m~7ement of the weighted mesnber 48 to the lef t in 3F IG . 2 is limited by the ciosed : - - .
erld 46 o~ the c:ham~er 44. sillce the weighted member 48 : `
moves in response to veh~cle deceleration, it will here~ .
~fter be re~erred ~o a~ the i~er~ia weight 48. .
Housing 20 also has a bore 53 of a smaller diame- :
ter than cavit~ 30 connecting the cha~ber 44 and the :
cavlty 30. Another bore 54 of a diame~er grea~er ~han bore S3 but smaller than c~vity 30 is provided inter mediate bore 53 ard cavity 30 and defines Qnnular shoulder8 /
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r 56 and 57 . The bores 38, 39 " 53 and 54 are disposed co-axially with xespect to each other and ko the cavity 300 A movable valve me~iber suoh as Yalve pistoll 58 ~ :-:
is disposed in the cavity 30 and has its uppe~ aIad lower ~``~r~
eD~d portions 59 and 60 slidably posi~cioned in the bores 38 and 53 9 respectively. An 0-ring seal 61 is proYided in the bore 54 contacting the annular shoulder 57 and the valve piston lower end portion 60 to prevent ~luid ~
flow rom the cavity 30 to the chamber 44. With the 0- :
xing seal 61 in position, i'c should be noted that the chamber 44 will be kept :Eree o~ brake fluid and will be :
maintained at atmospher~c pressure. ~owever, i~ is pos~ible that a small amount o ~luid could leak past ~ the 0-r~g seal 61 downwardly iIlto the cha~er 4~ ox ~ .
eæampl~, the seal becomes worn~ Therefore, 1 de~ired, ~ :
a well~ owrl weep ~alve (not shown) could be installed i~ the chamber 44 to exhau5t any :Leaked fluid to the . -:
atmosphere. . ~ ~ .
The valve- pistoI 58 includes a radialIy ex~end~g -annular fla~ge 62 having all annular shoulder 63 oll the . - ~
bot~om side and an anr~ular shoulder 64 on the top side .: ~ ~-thereo:e~ The valve piston 58 further ~cludes an eIllarged~ ~ ~ `
poxt~on 66 ea~tending radially outwardly- a~bo~ the annular -; ~ I
:~lange ~2. The enlarged portion 66 has 8 rounded ar~rlu1ar -shouldex 6~ which fvrms a valve headO The outside diame- -.
ter of ~he enlarged portion 66 ~ s ~Lightly less than the . ~ -:
diameter o the counterbore 39 of ~he eap 34 so as to allow fluid f:Low around the enlarged portion 66. Valve piston 58 also includes a reduced diam~te~ cyllndrical .
portlon 68 be~ween the annular shoulders 64 and 67 - . ~ ` ~`-` ? ,~

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Valve piston 58 has a fluld passage 69 e~tellding .
transversely thr~ugh the intermediate portion the~ o~
and a cavity 70 of circular cross-section extending .
lollgitudinally through the upper end portion 59 there~y S forming a peripherAl wall 65. A fluid passage 71 .
connects the fluid passage 69 and the eavity 70. A pair o fluid passages 72 extend from the cavity 70 through the periphexal wall 65 of the valve piston upper end -:
portion 59. ¦
A 3 ationary valve member ~uch as elastomeric -~:
valve element 73 is disposed in the cavity 30 a~d :
surrounds the reduced cylindrical portio~ 68 of valve pistorl 58. ~en the valve elemen~ 73 is ill the position show~ in FIG. 29 ~ engages the wall of the cavity 30. -~
A~ seen irl FI~S. 3 and 4, the valve element 73 has a depending lip 74 which extends downwardly and out:wardly when the valve ele~ent 73 is ~ 8 free ~tatc. When the valve element 73 is installed ln t:he cavity 30, the lip ~
7~ is forced inwardly a~d, conseque~tly, forms a seal ¦ .
against the wall of the c~vity 30 to pr~v~nt upward fluid flow around the llp 74 from t~ie cavîty 30 to the .
outlet opening 29, - . - :
The valve element 73 includes a plurality o .
angularly spaced xibs 76 which extend radially from the ;~
outer peripheral surface 77 thereof and contact the wall of the cavity 30 above the lip 74. Valve element 73 further includes a plurality o~ semispherical bosses 78 which extend downwardly from the lower side 79 thexeo . ¦
fox engagement `wl~h ~he annular shoulder 64 of valve ¦ `

3()piston S8. The spaces betwee3n ~he bosses 78` deine fluid `/ ~
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paSsacJes ~or upward -Eluid flow. Valve element 73 also lncludes a plurality of angularly spaced ribs 80 which e~tend upwardly from the upper si~e 81 thereof for contacting the bottom annular surface 41 of the cap 34. As seen in FIG. 4, the ribs 76 are angularly aligned with the ribs 80 to allow downward fluid ~low around the lip 74 from the outlet opening 29 to the cavity 30. As also seen in FIG. 4, the valve element 73 further includes a rounded portion 82 adjacent the upper end of its inner peripheral surface 83 for engaging the rounded annular shoulder 67 or valve head of the valve piston 58. The rounded portion 82 forms a valve seat.
When the control valve 18 is assembled as shown in FIG. 2, it will be seen that the diameter of the reduced portion 68 of the valve piston 58 is less than the diameter of the inner peripheral surface 83 of the valve element 73 so as to define an annular fluid passage therebetween. Also,':the outer diameter of the flange 62 of valve piston 58 is less than the inside diameter of the lip 74 of the valve element 73 thereby defining an annular fl~id passage therebetween. These . . . . . . . . . . . . . . . . . ..
annular fluid passages combined with the fluid passages between the bosses 78 o~ the valve element 73 de~ine a direct fIuid path for upward flow between the valve piston 58 and the valve element 73.
~he valve piston 58 is normally biased upwardly in FIG. 2 by a coil spring 84 so that a fluid passage is normally - formed between the val~e head 67 and the valve seat-82. The upper end of coil spring 84 engages the annular shoulder 63 on f lange 62 of the valve piston 58. The lower end of coil spring 84 engages a retaining riny sd/~ -10~

86. The retaining rlng ~6 rests on the annular 5houldex 56 o cavi~y 30 and surrounds the lowex end portion 60 .
of the valve piston 58. The retalning ring 86 maintains the ~oil sprin~ 84 in vertical alignment with the valve piston 58. .
An unrestricted ~luid passage is provi~ed through con~rol valve lB wi h the valve piston 58 in the position ~howrl in FIGô 2 sO 'chat the fluid pressure at the outlet opening ~9 will be equal to the fluid pressure at the : ~-~nlet ~penin~ 26 or in the master cylinder 11. T~e operatio~ o~ the valve pisto~ 58 ~nd the valve elemant 73 to provide a restricted fluid passage to the outlet opening 29 will be explalned hereafter. However~ it will : . ~
be noted that the coil spring 84 det2xmines at what :_.
1uid pressure level the valve piston 5S and the valve .
element 73 will begin to restrict the fLuid passage .
through the contxol valYe 18. This ~luid pr~ssure level .
i~ herea~ter re~erred to as the split point pressure and the coil spring 84 is hereaf~er referred to as the split `
~0 poin~ spring 84. -An L-shaped bracket 102 having holes 103 is .
attached to the sidQ o housing 20 or mounting the .
control val~e 18 on a vehicle.
,Y~LVE OPERATION, - 25 Referring now to FIGo 2, the control valve 18 is .~
~ounted on a vehicle with the arr~w X facin~ toward the ~ :
front of the vehicle and with the longitudinal axi~ 49 .
o the chamber 44 lying at an acute angle to the hor~zon- .
tal (herea~'cer referred to as the valve mount angle) so ~hat the open end 47 of the chamber 44 is a~ a lower f .'~
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z -1 level than the closed end 4~ thereof. I~ith the control .
valve 18 in this position, the iner~ia weight 48 re~ts ln a lrst posi~ion adjacent the inner e~d 52 of the plug 50 t~ereby obstructing the movement of the valve .
S piston 58. The valve mount angle determi~es at what v~hicle deceleration level the l~ertia we;ght 48 will move o~ard tGward the closed end 46 of the cham~er 44 .
~o all~w ree movement of the valve pis~on 58~ Thus~
~he level o~ vehicle deceleration which causes the inerkia weight 48 to move forward to~ard the closed end 46 of the .
chamber 44 is preselected and depends on the amplitNde .
o the valve mount angle. : ~;
The, Yalve mount angle may be varied in amplltude to achisve the desired movement o~ thP iner~ia wei~ht 480 ¦
lS For example, if the valve mount angle is relatively small, :
the ~nertia weight 48 will move fo~ard at a low2r level o~ vehicle decelerat~on than i~ the valve mount angle is relat~vely ~.arge. Alternatlvely, in lieu o the valve mount angle, the control valve 18 may be mounted on a ~-vehicle with the lo~gitudinal a~is 4g o~ the cha~ber 44 . l~ring parallel to the horizontal O The control valv e 18 would the~ include a coll spxing ~not shown3 in the chaniber 44 ~ormally biasing th~ inertia weight 48 toward .' the in~er end S2 of the plug 500 The coil spring would .
provide the same ~nc~ion as the valve mo~t angle and ;::
~uld be varied ln compre~ive force to achieve the ` ::
desired movement o the inertia weight 4~. :~
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UNLA~EN VEHIC~E C~NDITION
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During deceleration due to a brc~lng application ~
of the ve~icle when ~nloaded or lightly loaded (hereafter ~
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a22 ( referre~ to as the unladen vehicle~, the inertia weight .
48 rests ln lts Xirst position against the inner end 52 o the plug 50, as seen irl FIG.-2, until the preselected .;
level of deceleration is reached . When the unladen vehicle attains this preselec ted deceleration level, the inertia weight 48 will begin to move forward to~ard the :
. closed end 46 o~ chamber 44 before the fluid pressure at the inlet opening 26 attains the predetermined level known .
as the spllt point pressuxeO As the unladen vehicle .:
11) exceeds the preselec ed deceleration level3 the inertia . -~
weight 48 continues to move forward to a second position . :
adJacent the closed end ~6 of the chamber 44. The-fluid ~ ¦
pressure in the mastex cylinder 11 requixed for the un-laden vehicle to reach the presellected deceleration level is kn~wn as the urlladen actuation pressure. Thus, :Eor an unladen ~ehicle, the unladen actu,ation pxessure must be less tharl the spliS point pressurls ~ order for the con- ~ .
trol valva 18 to limit the ~luid pressure delivsred to .-.
the rear brake wheel cyl~ders 16a, 16b. `
~he corltrol. valve 18 provides a path for the .
direct transmission of ~luid pressu~e from ~he inlet .
ope~ing 26 to the outlet opeIling 29 when the valve piston - I
58 is ~n the positlo~ shown in FIG. 2 as already de5cr~bed" .
..
I)uring a braking application on the Imladen vehiclea this path remains open with the pre~sure at ~he outlet opening .
29 inereas~g at the same rate as the pressure at the :
inlet opening 26 until the predetenmined split point pres~ure.ls reached at the inlet opening 26 as represented by the l~ne connecting ~he points 104 and 106 in FIG. 9.
Before reaching the predetermined split point pressure . .
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r at inlet opening 26, the unladen actuation pressure is .
attained a~d produces the preselected level of decelera~
tion so that the` inertia welght 48 move~ forward to ~t.s ~eeond position ad~acent the closed end bf6 of the chamber ~
44. Whe~ the fluid pressure deli~rered to the inlet open- . ``.
ing 26 is then increased due to the continued braking application and attaills the predetermined split point value represented b~ the point 106 in FI&o 9~ the valve piston 58 moves downward against the force of the split ~-.
point spring 84 to a position where the valve head 67 of the valve piston 5~ contacts the valve seat 82 o~ the .~ y alve men~ber 73 and t~us closes the ~luid pathO There-after, the relatlonship be~wee~ ~he pressure at outlet opening 29 and the pressure at inlet openil~g 26 will be .` ~ -. generally represented by the line connecting the poin~
106 and 107 in FIG. 9~, P2 hereina~ter reer~ to ~he pressure at the outlet opening 29. Pl hereinafter refers to the pressure at the ~nlet ope~i~g 26 or th~ pre~sure .
~ the master cyl~der 11.
The predetermined le~7el of split point pressure is dependent upon the ~orce of the split poi~lt spri3~g 84 !:
~- compared ~o the ffective area of the valve piston 58 acted upon by 1uid pressure in a dire~.tion opposing ~he -- . oxce of the split po~t spxing 840 This effective area -.
2S is equal to ~he cross-sec:tiorlal area of ~he valve pis~on lower end portio~ 60 since the bo~om end o that portion ~s sealed o from the ~:nlet 1uid pre~sure (Pl) by the ~
~ring seal 61 while fluid pressure acts against all of :~ -the rema~i~g portions of ~he valve plston 5~. During ~ ~ . j . the lower r~ges of master cylinder pressure ~Pl3, the f `
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fluid pxessure acting on the e:Efectlve area of the valve .
piston 58 produces a downward force which is insu~fic~ent to overcome the force o the split point spring 8,4.
Assuming that Pl equals the fluid pressure at the inlet .
S openin~ 269 A equals the cross-sectlonal axea of the ~
valve plston end portion 60, and S equals the force of .
~che spli~ point ~pring 84, then the valve piston 58 will close the fluid path (that is, va~ve head 67 will move into engagement-with valve element seat 82) when PL times A ~s greater than Sc - -~
The operation of the control valve 18 ~co regulate the 1u~d ~low between the inlet opening 26 and the outlet ` .~-:
opening 29 will now be e~plainedO. This operation of the control valve 18 is represented by the line ~onnect~ng ¦
the points 106 and 107 in FIG. 9~, Aiter the valve head 67 c:loses ~ainst the valve element seat 82 alld the ~luid pressure at the inlet open-lng 26 is further ~ncreased by the master cylinder 119 the ~ncreased level of ~luid pressure will act aga~nstl - .
the valv~ plston 58 over 2n effective circular area hav ing a diameter equal to the main sealing diameter of the valve head 67 (hereinafter referred to as B) less the .
area A. This produ^es an upward force on the valve piStOll :
58 assisti~g the split point spring 84 and tending to reopen the valve piston-58 to deli~rer at least a portion of this increased fluid pressure ~o the outlet opening 290 ~loweYer~ any of this ~ncreased flu~d pressure - . delivered to the outlet opening 29 creates an oppos~ng :
downw~rd forc~ on ~he valve p~ston 58 acting ove~ the ~:
area ~. This~ of course, tends ~co reclose ~he valve ~
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. . rpiston 58 aga-inst the valve element 73~ - .
These opposing forces on valve piston 5~ tend to keep the valve head 67 closely adjacent to the valve seat .-82 for t~e restricted flow o 1uid ~rom the inlet apeni~g 26 to tke outlet ope~ing 29 to create a pressure at the :
outlet opening 29 which increases at a lower ra~e tha~ the pressure at the i~let opening 26~ The ratio of the pxes~
sures ~Pl/P2) is determined by ~he rela~ionship of the effective areas (A and B) previously me~tioned. Af~er the control valve 18 ~irst closes (iOe. valve head 67 iRi-tially engages valve element seat 82~, the increase ~n pressure at the inl~t opening 25 (hereinatex referred to as ~ Pl~ w:Lll produce an incxease~ in pressure at: the outlet open~ng 29 (hereinaf~er rel erred to as ~ P~
accordance with.the following formula: A Pl BJ3A 3 A P2.
Similarly, if we assume P~! is the pressuxe at the outlet opening 29 ~b~P2(B) ~ Pl~B-A) ~ S after the valve piston 58 becomes operative (i~e. ater the valve h~ad 67 engages valve sea~ $2).- Accordingly, the fluid pressure . :~
exis~g in the fr~nt brake wheel cylind~rs 13a~ 13b will be greater than the fluid pressure existing in the rear . ¦
brake wheel cyl~ders 16a~ 16b when the ma~cer cylinder .~ ~ ~
- pressure Pl surpasses the unladen actuat~on pxessure ~ ~:
a~d then exceeds the split po~ pressureO ~.
- 25 During that por~on of a bra~;e application in ;~
which the applied pedal e~fort ~s reduced ~ub~equent ~:
a pedal effort o sufficie~t :intensity ~o have moved the valve piston 58 to the xe~tricted 1s;~w po~tion7 Pl act~ng on the are~ B-A is reduced. Thus~ tha forces ~ending to ~ .
~ove the valve pi~ton 58 upwardly toward the outleC ~ ~ ~

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opening 29 are xeduced and the val~7e piston 58 moves .
downwardly under the inf luence of the pressure at the ou'clet opening 29 (P2) acting on the area B. As the valve piston 58 moves downwar~, its valve head 67 slides with~n the inner peripheral surface 8~ of the valve element 73, thereby increasing the available volume for the ~luid at the rear brake wheel cylinders 16a, 16b and thereby a~complish~ng a reduction in P2. The valve piston 58 wi:ll contillue its downward movelQent in an effort to reduce P2 to satisfy the previously mentioned formula: ~
P2(B) - P~ A) ~ 50 :
However, the valve piston 58 may not completely ~, achieve this result because o the limited possible down~
ward travel thereof. Dur~g the descen~ :In brake pres~ . .
sure, the pressure at the outlet opening 29 can never be greater than the pressure at the :;nlet opening ~60 This is because the fluld at the outlet op~ning 29 is able to . .
flow downwardly arou~d the valve el~ment 73 between the :l~p 74 and 'she walL~of the cavity 3~ if the fluid prPssure ~0 in t~e cavity 30 is at a lower l~vel7 The lip 74 of the val~7e element 73 accordingly functions as a relief valve to prevent rear brake pressuxe (P2 at outlet opening 29) ~rom ever being greater than front brake pressure (Pl at ~ .
master cylinder 11 or inlet opening 26)o ~5 Whe~ the pressure at the inlet opening 26 is reduced beneath the level at which ~he control valve 18 originally closed, the valve piston 58 will be moved .
upwardly bsr ~che spl~t poin~ spring 84 ~co reop~n the fluid passage between the val~e head 67 and valve seat 82. Therea~er, whe.n vehicle deceleration no longer ,' ,~

- 17- .

- ( . ( ``~`:

. ~ . r exceeds the original preselected leve~ at which the inex- .
tia weight 48 moved ~o~ard, the valve mount angle will cause inertia weight 48 to re~urn to its first position.
It should ~ow be apparent that when the control -valve lB is mounted on an unladen vehicle, the inertia .~
weight 48 will respond to a preselected level of vehicle deceleration and move forward toward the closed end 46 of the chamber 44 before the pressure at the inlet opening 26 reaches the predetermined value known as the split poin~ Prior to the inlet press~re reaching the spli~ .~
point value, there is direct communication betwee~ the --.
i~let opening 26 and the outlet ope~ing ~9. When the i~-let prèssure exceeds the split point value? the control valv~ 18 limits the pressure at the outlet opening 29 pxovided that the inertia weight 48 has moved toward and xast~ ad;acent the closed end 46 of the chamber 44. `
During the portion o~ a braking applica~ion o~ .
th~ unladen veh~.cle subsequent to the pressure at the inlet opening 26 reaching the split po~nt value~ ce~tain s~tuations such as brake ~ade or wet ~rake~ which require ¦ -~
~ncreased rear brake pressure may arise. Accordingly, - the control valve 18 may include a by-pas3 valve assem-. bly which operates to increase ~he pressure in ~he rear bra~e wheel cylinders 16a, 16b after a period when the pressure delivered to the rear brake wheeL :
cylirlders 16a, 16b- is limited" r - - Referr~ng now to FIGS., 2, 2a and 7, the by-pass valve as9embly is disposed in the cavity 70 of the valve :: piston 58 and includes.a check valve member B8 havlng c~ -:
outer surface of he~agonal configuratLon, The check valve - 18 ~ "
...- :
, ~

- .

member ~8 has a eavity 89 at o~e end and a stem 90 at the . .
other end~ The by-pass valve assembly also ineludes a seal member 9I positioned in the cavity 89 of the check valve member 88~ The by-pass valve assembly urther includes a coil spring 92 engaging the check valYe member 88a A xe~
taining cap 93 is disposed in the upper end of the valve piStOll cavity 70 and is secured therein by bending the end 94 of the peripheral wall 65 inwardly to contact the cap 93. When the by-pass valve assembly is installed in the valve piston 58,.ths coil spring 92 is placed in compres-sion thereby normally biasing the check valve member 88 a~d the seal member 91 downwardly together to a closed position wherein an annular portion o the seal member 91 is in engagement with the bottom o:~ the cavity 70 so as ~o lS prevent ~luid flow between the pas'sage 71 and the caYity 70.
When tha valve piston 58 and the valve element 73 establish a predetexmined pressure differential between .
the inlet pressure Pl and the outlet pressure P2 which is represented by the pressure differe~ce between the points 108 and 107 in FIG. 93 the eontrol valve 18 - operate~ in response to increasing inlet pressure Pl to provide a ~y-pass ~luld path through ~he valve piston .
58 for the transmission of fluid pressure from the inlet opening 26 to the outlet openin~ 29. This operation of ~5 the control valve 18 is represented by the line through the points 107 and 109 in FIG. 9 as now described. ~~
The predetermined pressure differential between the inlet pressure Pl and the outlet pressure P~ is dependent upon the force of the spring 92, the efec~ive area o the seal member 91 acted upon by the inlet ., . :
.- 19- ;
' :' , .

(~ ~lOlQZ2( ~ ~

pre~sure Pl, and the efectlve area of the seal mem~er r~
91 acted upon by the outlet pressure P20 Each of these .
effective areas is equal to the circulax area having a diameter equal to the outside diameter of the annular :.
S portion o~ the seal member 91 whieh engages the bottom o the cavity,70. Assuming that C equals thls ef~ective circular area and Sl equals the force of the 5pring 92, --:
the check valve member 88 and the seal member 91 wtll move upward together to an open position (i~ e~ the seal : ~-member 91 will be spaced from the bottom of the cavity 70 so that the passage 71 will be in fluid communication ~ith the cavity 70) when P~ acting on the area C becomes -~
greater than P2 acting on the area C plu~ the force S10 1 ~
~ Fluid then ~lows through the pasfiages 69, 71 in~o thc. . 1 ~`
15 cavit~ 70 and upwardly around the outer heæagonal surace ¦
of the check valve member 88 through the passages 72~ 42, ¦
32 to 'che outlet operling 29 thus inc~easing the fluid ¦
pressure ~t the outlet opening 290 The forc2 developed by the increa~ed outlet pressure P2 actin~ o~ the valve ¦ ;
pist~ 58 over the area. ~ isnmediately overcomes the force developed by the ~let pressure Pl acting on the valve piston 58 over the area (B~A) plus the force S o I -the split point spring 84~ Consequently, the valve :. I
piston 58 will close against thP ~alve elemènt 73 (iOeO ~ -valve head 67 will move into engagement with val~e element seat 82) and will remai~ closed against the valve ele~ent 73 a8 the inlet pressure increases thereaf~er. Thus, fluid ~low between the valve pist~n 58 a~d the valve ~ ~:
~lement 73 will be terminatedO ~ - :~
.. - 30 ~hen the increasing outlet pressure P~ reaches a `J
. ~ - i ~ 20 . . ;, .

llOlUZ~

value such that P2 acting on the area C plus tlle ~orce Sl :Ls equal to or greater than Pl acting on the area S::, the .
check valve member 88 and the seal me~bex 91 will move back downward i:ogether to the closed position (iOe~ the seal ~ember 91 will reengage the bottom of the cavity 70 to again prevenc fluid flow bet:ween the passage 71 and the c~vity 70)~ Wh~n the ~nle~ pressure is then increased so that Pl acting o~ the area C again overcomes P2 acting on ~he area C plus the force Sl, the check valve me~b r 8~
and the seal member 91 will move back upward together to the open position. This operation o~ the check valve mem- .
ber 88 and the seal member 91 will be repeated as the out-let pres5ure P2 tthe pressure deli.vered ~ the rear bralce wheel cylinder~ 16a, 16b) is increased in accordance with the llne 107~109 in FIG., 9O . .
I~ the braking applieation is discontinued subse~
~uent to the check valve member 88 and the æeal mem~çr 91 l~itially moving to the open posit:ion, the inlet pressure Pl act mg on the area C will be reduced and ~he predeter-mined pressure differential be~ween the inlet pressu~e aQd the outlet pressure will no longer exist. Consequently3 .
th~ check valve meaiber 88 and the ~eal m~mber 91 will move to the closed position ~ince the outlet pressure P2 acti~g .
on the area C plus the foree Sl will overco~e the inlet pre~sure Pl ac~ing on the area C~ ~he outLet pressure P2 will be reduced as previou~ly described by the sliding . _ of the valve head 67 of ~he valve piston 58 inside the lnner pes~ipheral surface 83 of the valvé element 73 and I \
by the downwa~d flow of 1uid between the lip 74 o~ the valve element 73 and the wall of the cavity 30~ This will prevent the outlet pressure P2 ~rear brake pres~ure~ ~rom ., . - :.
:' .. `:
- .. ~ ~

being ~reater than the inle-t pressure Pl (~ront brake pressure) during decreasin~ inlet pressure.
It should now be apparent that once the predetermined pressure differen-tial is established between the inlet pressure and the outlet pressure, th~ check valve member 88 and the seal member 91 will reciprocate between the open and closed positions thereof in response to further increases in the inlet pressure while the valve piston 58 remains closed against the ~alve element 73. This operation of the control valve 18 as represented by the line 107~109 in FIG. 9 will create an outlet pressure P2 which increases at the same rate as the inlet pressure P1 and will maintain the predetermined pressure differential between the inlet pressure P1 and the outlet pressure P2, The outlet and inlet pressures increase at the same rate along the line 107-109 since the effective areas of the seal member 91 acted upon by the :inlet and outlet pressures, respectively, are equal. Along the line 107-109, the outlet pressure P2 will be determined by the formula p PI: 1 The by-pass valve assembly thus provides the capability for the vehicle operator to override the unladen mode of limiting the rear brake pressure in those situations which require increased brakin~ torque.
During the early stages of a braking application on the unladen vehicle prior to the vehicle reaching the pre-selected deceleration level (i.e. prior to the pressure in the -- master cylinder 11 reaching the unladen actuation pressure), surges of fluid pressure at the inlet opening 26 of the control valve 18 may tend to move the valve piston 58 downward prematurely and thus inadvertently trap sd/,~ 22-. .

Z2 ( .

the inertia weight 48 in its first position adJacent the r-inner end 52 of the plug 50~ This would then prevent the .
valve piston 58 ~rom moving freely to llmit the fluid pres-sure at the outlet opening 29 when the inlet pressure ex-ceeds the split point value. Accordingly~ the contxol valve 18 ma~ include a~ anti-spike valve assembly whlch becomes operational to preventpremature do~nward movement o the valve piston 58 by momentarily restricting the 1uid ~low . .
from the inlet opening ~6 ~o the cavity 30 whenever the i~let opening 26 receives a predetermined level of high volume fluid flow. -Referring now to FIGS~- 2 and 8~ the anti-spike YalVe assembly is disposed in the bore 22 o~ the housing 20 and includes a spike piston 96 having an ou~er surface of hex- -:
agonal ~on~iguration. The spike piston 96 has a cavity 97, a~ end wall 98, and ~n annular en~ surface 100~ A restri~t- :`
ed fluid passage 99 extends through the end wall 98, and a .
notch (not 8hown) extends t~ansversely across the end wall 98 ~o allow ~luid flowv The spike piston 96 is normally -biased to the le~t i~ FIG. 2 by a coll spring 101. ~:
I~ a pressure spike in the brake ~ystem results .
in the pre~etermined level o~ high volume fluid flow at the inlet opening 26 of the control va~ve 18~- a pressure drop ~ill be created across the end ~all 9~ o~ ~he spike pisto~ 960 This pressure drop wiil be suficient to move the spike pisto~ 96 to the right in FIG. 2 against the force of the spring 101 until the annular end sur~ace 100 ~ .
thereo~ enga~es the inner end of the bore 2Zo Thi5 will xestric~ fur~her 1uid ~low around the outer hexagonal :
surface o the spike piston 96. The CaYit~ 30 will then , ' '- : .. ~
, ~ ~
23 - . ~ ~ -' ..`' . i, -~ ( ( ~:
:~O~ Z2 rece:Lve a ~rolume of fluid which is su~stantially smaller than the predetermined high volume level through the re- .
stricted passage 99 of the ~pike piston 96. The pressure from the substantialLy smaller volume of 1u:Ld acting on .
S the valve piston 58 over the area A will not be suffic~ent - to move the valve piston 58 downward agaiIlst the force ofthe split point spring 84. When the volume of fluid 10w at the inlet opening 26 subsequently decreases below the predetermined high volume level, the pressu~e drop will be ~ :
reduced and the spike piston 96 will be biased to the let ~n FIG. 2 bg the coil~sprin~ lOl thereby reestablishing 1uid ~low around the outer hexagonal surfaee of the spike piston 96. me anti-spike val~e assemhly thus minimizes .
the possibillty of lnadvertently trapping the inertia weight 1~ b,8 i~ its fixst positlon when stopping the unlade~ vehicle~ ;
~ ~DI,TI, N .
I~ the same vehicle is ~w loaded in e~cess of a pre~ .
selected weight (hereinafter re~erred to as the laden vehlcle~
the laden actuation pressure ~iOe~ the fluid pressure i~ -the master cylinder il requixed fo~ the lade~ vehicle to :~
..
: reach the preselec~ed level of decelera~ion due to a braking applicat~on) w~ll be greater than both the unladen actuation.
pressure ~nd ~he split point pressure~ In designing the co~trol valve 18, a deceleration level i5 chosen at which : :
rear brake pressure limiting is ~o be ini~iated on the un~
laden.vehicleO Then the unladen and the lad~n actuatlon pressures are determined. I.astly, the split point pressure is designed to be greater than the unladen actuat~on pres- ¦ -~ure, but l~ss than the laden actuation pressure. The: i operation o control valve 18 on an unladen vehicle where the sp~:it point pxessure ~ s greater than the unladen ~ .

'~ '~:'.f, 2 ~ `

actuation pressure has already keen fully describedO
During deceleration of the laden vehicle due to a braking application, the inertia weight 48 will remain in its first position adjacent the inner end 52 of the cap 50 as seen in FIG. 2 even th~ugh the preselected level - of deceleration may be reached and exceeded. This is accomplished because the fluid pressure at the inlet opening 26 attains the split point val~e before the laden actuation pressure ls reached and produces the preselected level of deceleration. When the split point pressure is reached, the valve piston 58 moves down~ard slightly and contacts the inertia weight 4~ which has rèmained in îts -first position because the preselected level of decelexa-tion has not been reachedO The physical contact between the valve piston 58 and the inertla weight 48 traps the inertia weight 48 in its first position and prevents ¦
forward movement of the inextia weigh~ 48 a~ decelPration l~vels above the preselected deceleration level. Further downward movement of th~ valve piston 58 is obstructed by the inertia weight 48 thereby preventing the valve ¦
: piston 58 from closing the fluid path through the contxol valve 18 (i~e. valv~ head 67 does no~ engage valve seat 82). This results in the pre~suxe at the outlet openin~
29 increasing at the same rate as the pressure at the ~5 ~nlet opening 26 for the full range of inlet pressure as ., shown by ~he line connecting points 104, 106~ 108 and lOS
~n FIGo 9 sinee the valve piston 58 is disabled 1n a . J
posltion such that the fluid path through the control valve 18 remains open. Consequently, the pressure delivered 3û to the rear brake whee1 cy1inders 16a, 16b wi~l be equal . :
to the pressure delivered to the front bralce ~wheel `

~ 25 ~ ` ~-. '`' ~J' ' ¦ .`' .

`' (`' (;`

. . r cylinders 13a, 13b~, .
In summary? the inertia weight 48 provides a ~:
means for automaticall.y disabling thP valve piston 58 .
depending upon the vehicle loading condilt~on. The. inert~
weight 48 is movable from a first position wherein it obstructs the normal movement o~ the valve piston 58 and - dlsables the valve piston 58 in a position such that the ~:~
fluid path through the control valve 18 remains open to -:~
a second position where;n the valve piston 58 is frëely :~
movable to open and close the fluid path through the - ~ -control valve 18. The movement of the inertia weight 48 ~-:
~xom its first position to its second position is depen-~::
dent upon the loading of the vehicle~ If the veh:lcle is~.~
unloaded or lightly loaded, the inertia weight 48 wlll : -move to its second position ~hereby not disabling or ~: -obstructing the normal movement of the valve piston 58.
However~ if ~he vehicle is loaded in excess of a pre-selected weight~ the inertia weight 48 will rema~n in its ~-- . first position thereby disabling the valve piston 58 in -a position such that the fluid path through the control ¦
valve 18 will remain open. . .
~ COND EMBODIMENT -:
.
Referring now to FIG. 10, a second embodiment of the control valve, indicated generally at 18a9 ls 9h for use in the brake system portion of FIGo 1 iII the same location as the previously described control valve 18~
- The con~rol valve 18a has substan~ially the same par'cs as the control valve 18 except for the parts now described.
.
The housing 20a of ~che control valve 18a has a~.
second cavity llQ adjacen~ the c~tral cavity 30. Fluid ` ~ ! ' .
.... ~ ' ` ~ . ,. . , `~

-passages 111 and 112 connect the lower portion of the cavity 30 with the cavity 110. A fluid passage 113 .
- connects the upper portion of the cavity 30 with the -cavity 1105 and a fluid passage 114 connects the caYity llO wlth the outlet opening 29. The valve piston 58a of the control valve 18a is substantially the same as the valve piston 58 of the control valve 18 but without the fluid passages 69 and 7~, the cavity 70, the fluid pass~
ages 72 9 and the by-pass valve assemhly in the cavity 70O
lO ~ The control valve 18a has ~ WX~#~ blend-back :
- type o by pass valve assembly which is disposed in the cavity 110 of the housing 20a and includes a check valve .
member 116 having an outer surface of hexagonal config-uration. The check valva member 116 h~s a stem 117 at :
: 15 one end and a cavity 118 at the other end. This blend-back type of by-pass valve assembly also includes a seal member 119 positioned in the cavity 118 of the check valve member 116~ A co~l spring 120 is disposed around the stem 117 of the check valve member 116 and engages the lower 20 - portion of the check valve member 1160 A guide- member 121 is disposed in the cavity 110 a~d is held therein by a retainer ring 122. The stem 117 slidably extends through a central hole în the guide member 121. A pro-tective cap 123 covers the ~op of the guide member 121.
~5 An 0-ring seal 124 is disposed between the guide member 121 and the cavity llO, and an 0-ring seal 126 is disposed between the guide member 1~1 and the stem 117. When this blend-back type of by-pass valve assembly ~s lnskalled in the cavity llO, the coil spring 120 is placed in compres-sion thereby normally biasing the check val~e member 116 - - ~ ` . , i . ~
- , : .' .... ; ;~ ~'``.'`
, . . . .

2 .

and the seal member 119 downwardly together tv a closed .
position wherein an annular portion of the seal member 119 is in engagement with the bottom of the cavity 110 . ., 80 as to prevent ~luid flow between the passage 112 and :
S the cavity 110. ~:
During a braking application on an ullladen vehicle, the control valve 18a operates in the same manner as tha control valve 18 to limit the f luid pressure deLivered f:Q
the rear brake wheel cylinders 16a, 16b until a pxedeter-mined pressure differential has been attained between the. .
~:
. pressure ~Pl) at the inlet opening 26 and the pressure :
(P2) at the outlet opening 290 When ~he predetermined .
pxessure diferential between the inlet pxessure Pl and . , .
` the outlet pressure P2 which is r epresented by the pres-sure diference between the points 108 and 107 in FIG. 9 :~
has been established, ~he con~roi. ~alve 18a operates in response to increasing inlet pressure Pl to increase the ,`-pressure 1~ the rear brake wheel cyl~nders 16a, 16b until ~ ~ :
- it is again equal to the pressure in the fxo~t brake wheel ~0 cyl~ders 13a, 13b by providing a by-pass fluid path ¦
~hrough the cavity 110 o the housi~g 20a.. This operatiQn I :
o the control valve 18a is represented by the line through .the points 107 and 105 in FIG~ 9 as now descxibed. ...
The predetermined pressure differential be~ween the inlet pressure Pl and the outlet pressure P2 which is required fox pxoviding ~he by-pass fluid path through the cavity 110 o the housing 20a is dependent upon the force of ~he spring 120 (hereafter referred to as S2), the ~ffective area of ~h seal member 119 acted upon by the ~nlet pressure Pl, and the effective area of the seal ~

~ 8 ~ ~!
~. : . . ~

," ,. .. , .. , , .. = .. .. . ....... . .. .. .. .

~`` ( member 119 acted UpOIl by the outlet pressuxe P~, The ~ .
effective area o ~he seal member 119 acted upon by the inlet pressure Pl is equal to the cricular area (herea~ter referred to as D~ having a diameter equal to the outside diameter of the a~nular portion of the seal member 119 wh~ch engages the bottom of the cavity 110. The e~ective area o~ the seal member 119 acted upon by the outle~
pressure P2 is equal to the area D less the cxoss-sec~ional area o~ the stem 117 of the check valve member 116 (hereafter referred to as E) since the upper end : portion o~ the stem 117 is sealed off from the outlet pressure P2 by the 0-ring seal 126~ When Pl acting on the area D becomes greater than P2 acting on the area . :.
(D-E) plus the force S2, the chec,k valve member 116 and the seal member 119 will move upw,ard together to an open position where~n the seal member 119 is spaced from the bottom of the cavity 110 so that the passa~e 112 will be in fluid communication with the cavity 110. Fluid then flows through the passages 111, 112 Into the cavity llO -:
and upwardl~ around. the outer hexagonal surface of the check valve member 116 through the passage 114 to the out- -let opening 29 thus increasing the fluid pressure at the outlet opening 29. The force developed by the increased outlet pressure P2 act~ng on the valve piston 58a over :~5 the axea B immediately overcomes the orce developed by the lnlet pressure Pl acting on the va~ve piston 58a over the area B-A plus the ~orce S of the split point sprin~
840 Consequently, the valve plston 58a will close against the valve element 73 (iPe. valve head 67 will move into . eng~gement wlth valve seat 82) and will remàin cLosed .' ^`~
' :

2~ ~ D~

, .
... , . , '1 ~

2 ( `

against the valve element 73 as the inlet pressure in- r creases thereafter. Thus, fluid 10w between the valve .
piston 58a and the valve element 73 will be terminatedO
Whe~ the increasing outlet pressure P2 re~ches a ~alue . .
such that P2 acting ~ the area (D~E) plus the orce ~2 ~s .
equal to or greater than Pl acting o~ the area D, the check valve member 116 and the~seal member 119 will move back : -downward together to the closed position (i.e. the seal -member 119 will reengage the bottom of the cavity 110 to ~ again preve~t fluid flow between the passa~e 112 and the cavl~y ilO)~ Nhen ~he inlet pressure~hen increased so ~ ~
that Pl acting on the area D again overcomes P2 acting on .
t~e area (D-E) plus the force S2, the check valve member 116 and the seal member ll9 will move back upw æ d togethex to ~-the open positlonO This operation of ~he check valve mem-ber 116 and the seaL member 119 w:Lll be repeated as the ¦
o~tlet pressure P2 (the pressure delivered to the rear brake wheel cylinders 16a, 16b) is increased in accordance with ` .
the line 107-105 ~n FIG. 9 until ]P2 becomes equal ~o the ~ -~let pressure Pl tthe pressure delivered to the ~ront b~ake wheel cylinders 13a, ~3b).
When .the outlet pressure P~ eventually becomes equal . ~
to ~he inlet prPssure Pl as represented b~ the pQin~ 105 in ~ :.
FIG. 93 the check val~re mem~er 11~ and the seal me~er 119 ~ -:25 will move together ko the open position and wîll remai~ in the open position as the inlet pres~ure increases thereafter I .
~ince Pl acting on the area D ~ill be greatex ~han P2 ac~lng `
on the area (D-E) plus th~ f orce S2 . Thus, an open path is : - `
provide~d for the direct ~crarlsmission of 1uid pressure from: ~
, the inlet opening 26 to the outlet opening 29 at levels of :,; `~
inlet pressure gi:eater than the level repxesented by the ~ ~ r`~
point 105. `
. ~ 30 ~i . ,.~ ~

~L~O~
I~ the bra~in~ application is discontlnued subsequent to the check valve member 116 and the seal member 119 initially moving to the open position, the inlet pressure Pl acting on the area D will be reduced. Consequently, the check valve member 116 and the seal member 119 will move downward together to the closed position since the outlet pressure P2 acting on the area (D-E) plus ~he force S2 will overcome the inlet pressure Pl acting on the area D. The outlet pressure P2 will be reduced as previously described by the sliding of the valve head 67 of the valve piston 58a inside the inner peripheral surface 83 of the valve element 73 and by the downward flow of fluid between the lip 74 of the valve element 73 and the wall of the cavity 30, ~his will prevent the outlet pressure P2 (rear brake pressure) from being greater than the inlet pressure Pl (front brake pressure) during decreasing inlet pressure~
It should now be apparent that when the predetermined pressure differential is established between the inlet pressure and the outlet pressure, the check va].ve member 116 and the .. . . . . . ..
seal member 119 will reciprocate between the open and closed positions thereof in response to rurther increases in the inlet pressure until the outlet pressure becomes equal to the inlet pressure while the valve piston 58a remains closed a~ainst the valve element 73. This operation of the control valve 18a as represented by the line 107-105 in FIG. 9 will create an outlet -pressure P2 which increases at a higher rate than the lnlet pressure Pl until P2 becomes equal to Pl. The outlet pressure increases at a higher rate than the inlet pressure along the line 107-105 since the effective area sd/h, -31-- : ~:
r (D-E) of the seal member 11? acted upon by the outlet pressure P2 is less than the effective area D acted UpOII .
by the inle'c pr~ssure Pl. Along the line 107-195 the outlet pressure P2 will be determined by the formula Pl D ~ S2 P~ ~ (D E) c ~lusg the blend~back type of by pas s valve assembly provides the capabili~y ~or the vehicle operator to override the unladen mode o~ limiting the rear brake pressure and increase the rear brake pressure until it is again equal to the front brake pressurel Dur~ng a braking application on a laden v~hicle, the control valve 18a operates in the same man~er as the control valve 18 to raaintairl the fluid pressure delivered - to the rear brake wheel cylinders 16a, 16b equal to the pressure delivered to the front brakc wheel cylinders 13a, 13b.
THIRD EMBODI~NT
Referring now to FI&. 11, a third embodiment o the control valve, indicated generally at l~b, 1s shown fox use in the brake system portion o FIG. 1 in t~e same 2~) locat~on as ithe previously de~cribed co~trol valve 18.
The control valve 18b has substanti~lly the same parts :
a~ the control valve 18 except that it also includes the .
parts now descxibed.
The housing 20b of the control valve 18b has an I `~
open ended caYity 127 which is connected to the chamber 44 by a boxe 128~, l'he cavity 127 is connected to the lower portion of the central ca~7ity 30 by a ~luid passage 129. A hollow guide element 130 is disposed in the cavity 127 and is held th~ein by a xetainer rin~ ~31. ~ ~ .
A stop pl~nger 132 is slidably disposed in the hollow - 3~
,, ; ~-`':, 2~ ( guide element 130. ~ spr~n~ 133 biases the stop plugner 132 toward the right in FIG. 11 so that the e~d po~tion 134 thereof extends through the bore 128 into the chamber 44. An 0-ring seal 136 is positioned between the guide element 130 and the cavity 127, a~d an O~xing seal 137 is positioned be~een the stop plunger 132 and ~he guide element 130. ~nothex 0 ring seal 138 is disposed between . .
the s~op plunger 132 and the cavity 127. A retaining ring 139 prevents displacement of the 0-ring seal 138 and the hollow guide element 130.
The control valve 18b is mounted on a vehicle in the ~ame manner as the control valve 18 and operates in the same manner as the control valve 18 to control the ~luid pressure delivered to the rear brake wheel cylinders ~6a, 16b) during braking applicat:ions when the vehicle is in ei.ther the unladen condi~on or the laden conditionO
During brakes-of op~ratio~, vehicle bounce and I.
other vibrational e~fects may cause unwanted movement o~ the i~ertia weight 48 along the axis 49 of the chamber 44. Such movement is especiall~ undesirable in the laden ve~icle condition since the inertia~weight 48 may not be ~n position to prevent the valve piston 58 from clo~ing ths fluid path through the con~rol valve as p~eviou~ly described. As sho~ FIG. 119 the stop plunger 13~ of the control valve 18b is positioned to prevent movement of the inertia weight 48 along ~he axis 49 o~ the chamber 44 while the brake~ are of.
During a brak;ng application, the stop plunger 132 remains in the position shown in FIG~ 11 untll the .
pressure at the inlet opening 26 (P~ reaches a `
. ~

~ 33 - .

` !' ` '~ :' . ..

c :`

predetermined level. ~hen the i~let pxessure Pl reaches r this predetermined level~ the stop plunger 132 will move ~o the lef~ in FIG. 11 against the force o~ the spring 133 until the end portlo~ 134 thereof no longer extends S into the cavity 44. The inertl~ weight 48 may ~hen move from its irst pos~ ion to its second position~
The pxedetermined inle~ pressure level xequired to ully retrac~ ~he stop plunger 132 is, o course, . dependent upon the effective area of the stop plung~r 132 acted upon by the inlet pxessur~ Pl and the foree of the spring 133. The control valve 18b is designed such that . :
the pressure required to fully retract the stop plunger 132 is less than the previously descxibed split point pressure and lies somewhere a~ ong the li~e connecting the points 104 and 106 in FIG. 9 below the po~nt 1060 Thi9 :
i~ partlcularly impoxtant in the ~mladen vehi~le COI~itiO~ :~
~ince the inertia weight 48 must move to it9 second posi- .
tion before the split point pressure is reached for proper . operation of the csntrol valve 18b.
~ 1`
Reerring no~ to FIG. 12; ~ portion of a diagonal ~`
split brake system 1~ generally illus~rated at 140 and !:
includes standard componen~s sueh as a tandem master cylinder 141 operated by a bra~e pedal 142. The brake sy~tem poxtion 140 also includes front brake wheel cylinders 143, 144 and rear br ~ e wheel cylinders 146, 147. A fourth embodiment of the novel control valYe~
indicated generally at 148, is interposed between the master cylinder 141 and the front and rear brake wheel ~:
cylinder~ 14~, l44, 146; 147. Brake iluid is delivered 34 ~ -` ` ,~' ,: ~ ~

r fxom the master cylindex 141 to the cont:rol valve 148 via eonduits 149, 150 ~, Brake f luid is thPn delivered to the front brake wheel cylinders 143 3 144 from the control valve 148 via conduits 151~ 152. Likewise, brake ~luid is-deliYered to the rear brake wheel cylinders 146, 147 from the control ~alve 148 ~ia ~onduits 154~ 1530 -The control valve 148 includes a housing 156, preferably foxmed of a suitable metal, having inlet openings 1573 158 for receiving conduits 149, 150, res~
pectively, and outlet openings 159, 160, 161, 162 for reoeiving conduits 151, 152, 153; 154j respectively. The housing 156 includes a bore 164 and axially align~ ~
counterbores 165, 166, 167. A crossbore and counterbore -168, 169 is formed in the housing 156. A shoulder 171 is ~ :
foxmed between the counterbores 165, 166. A fluid passage :
17~ conne~ts the inlet opening 157 and the counterbore 166, and a 1uid passag~ 173 connects the inlet opening .
158 and the counterboxe 165.
The control valve 148 may incorporate a warning valve assembly, preferably of a well-~nown type which is similar to that disclosed in U. S. Patent No. 3,700,286, assigned to the assignee of this application. The warning valve assembly include~ an electrical switch 1743 a cen-tering piston 176, and a switch actuating piston 177O The electrical switch 174 is xeceived in the counterbore 169 ,:
and includes an operating member 178 which extends thxough the crossbore 168. The centering piston 176 is slidably - disposed in the counterbore 1660 An O-ring seal 179 is also slidably disposed in the counterbore 166 and engages one end of the centering pist~n 176~ The switch actuating ~ .

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:

. r-plston 177 is slidably received in the bore 164 and the ~-counterbore 165~ A plug 180 having a bore 181 and coun- .
terbore 182 is received in the counterbores 16G~ 167 and '~ is retained therein by a retainer ring 183~ The plug l~S~
carries O-ring seals 184, 185 in sealing engagemenk with the coun terbores 166, 167 . Passages 186 extend through the wal-ls of the plug 180. O-ring seals 187, 188, 189 axe carried by the switch actuating plston 177 in sealing engagement with the bore 1647 counterbore 165 and plug countérbore 182, respectively. .
The housing 155 also includes identical axially aligned cavities 191, 192 having outer threaded portions .
193, 194. ~assages 196, 197 connec~ the counterbores 167~ ~:
166, respectiveiy, to the cavity l91o Pa~sage~ 198, 19 connect the counterbore 165 and bore 164, respectively~
to the cavity 192. Passages 201, 202 connect the cavity 191 and the outlet opening~ 161, 159, respectively.
Passage~ 2039 204 connect the cavity 1~2 and ~he outle~ . :
openings 160, 162, respectively. A cyli~ rical chamb~r .
206 is provided in the housing 156 intermediate the -cavit~es 191, 192 and has a clo~ed end 207 and an open end 208. ~ores 209, 210 connec~ the cham~er 206 a~d the :
- cavities 191, 192, respectively. An inertia weight 212 `
which is similar to the previously described inertia .
~eight 48 is disposed in the chamber 206 in a firs~ posi-tion adjacen~ the open end 208 thereof for rollin~ or sliding movement toward the closed end ~07 thereo in response to vehicle deceleration. A plug mPmber X13 i~
disposed in the open end 20$ of the chamber 206. ~ l~s shown .
- `~n FIG~ ~3, the plug member 213 has a pair o a~ms 214~ `/

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ZZ ( 215 which provide co~cave suraces for guiding the rvlling or sliding movement o~ the inertia weight 212.
Valve pistons 217, 218 which are si~ilar to the previously described valve piston 58a are disposed ln the cavities 191, 192. Valve pistons 217, 218 are îdenticalO
As shown in FIGo l~a, each valve piston-217, 218 lncludes ~d portions 219, 221, an an~ular shoulder 222~ a rounded annular shoulder or valve head 223, a flange 224, a reduced c~lindrical portion 226, and an end extension 227~ Caps .
228, 229 which are identical are threadedly received in the threaded portions 193, 194 of the cavities 191, 192. :::
As shown in FIG. 12a, caps 228, 229 each include a bore . :-:
231, a counter~ore 232, an annular end sur~ace 233 and passages 234~ 0-ring seals 244, 245 are disposed be~ween lS the caps 228, 229 and the housing 156.
Elastomeric valve elements 236, 237 which ar~ `:
similar to the previously described valve element 73 are disposed in t~e cavities 191? 192 ar~und the reduced `
cylindrical portions 2269 bf the valve pistons 217, 2180 Wh~n the valve elements 236, 237 are in the poSit~ohS shown in FIG. 129 they engage the walls o~ the respective cavity ~ ¦
191~ 192 and the annular end surface 233 of the ~espec~ive cap 228, 229.
A substantially U-shaped leaf spring 240 is dis-posed in the chambex 206 and includes an intermediate portion 241 and resilient arm portions 242, 2430 The resilient arm portions 242, 243 engage the end extensions 227 of the valve pistons 217, 218. The lea~ spring 240.
thus normally biases the valve pistons 217, 218 into the positions shown in FIG, 12 whèreln the annular shoulders ,:
. ' ' ' ' . '~
. ~ 37 - ::~

: ' , . , .' " `'' .
222 thereof enOage the ;.nner end ~ rfaces of the cap counte~borcs 232.
The control valve 148 is mounted on a ~ehicle with the arro~7 X ~cing toward the front of tl~e vehiclc and with the open end 208 of the chamber 206 lying ac a lower level relative to the horizontal th~l the closcd end 207 thereof. Upon actuation of the tand~m master cylinder 141, equal fluid pressure is delivered to the -inlet openings 157, 158 of the control valve 1480 It will be understood that these equal inlet pressuxes act on the efective areas o~ the switch actuating and een-tering pistons 176, 177 in a manner similar to that described in the U~ S. Patent NoO 3,700,286 in order to maintain the switch actuating piston 177 in tlle posîtion shown in FIGo 12O
. During a~l braking applica~ions, the fluid pressure at the outlet openings 1.59~ 160 o the con~rol valve 148 will increase at the saMe rate as the 1uid`
pressure at the inlet op~nings 157, 158 thcreof, Thus, the pressure delivered to the fxont bralce wheel cylindcrs 143, 144 will b~ equal to the pressure il~ the master cylinder 141 for the full range o~ the mastcr cylindcr : pressure.
Durin~ deceleratlon due to a brakin~ application when tlle vehicle is in the unladen condition~ pressure.
at tlle outlet openings 161, 162 o~ the control valve 148 wlll increase at th~ SaMe rate as tlle pressure at the inlet op~nings 157, 158 thereof un~il the inlet pressure reacles the predetermined split pOillt value. Prior to reacllin~ the split point pressure, the vehiclc reacll~s a . ' .
. ~ 3~ - .

:

. r preselected level of deceleration which causes the inertia wei~ht 2~2 to move to a second positioD adjacent the .
c1Osed end 207 o~ the chamber 206O When the inlet pxes~ :
~ure then attains thP predete~n~ned split point value7 ~: }
S the`valve pistons 217, 218 move simultaneously toward the chamber 206 against the force of the resilient arm por~
tions 242, ~43 of the leaf spxing 240 until the valYe heads 223, thereo~ engage the valve seats o~ the valve elements 236, 2370 Thereafter, the va:Lve pistons 217, 21~ ':
and ~he valve elements 236g 23~ will operate to ~imit ~e :-pressure at the outlet openings 161a 162 in subs~antially :~ -the same manner as the valve pistons 58a, 58 and the valve element~ 73 as previously described and represented b~ .
the linè 106-107 in FIGo 9~ Thus~ the pressure delivered 15 to the rear brake wheel cy1inders 1469 147 wiIl be reduced relatiye. to the pressure delivered to the front brake ~ :
wheel cylinders 143, 1440 `
During deceleration due to a braking application n the vehicle is in the laden condition~ the inertia .
weigh~ 212 will remai~ in its first position ~jacent the ¦
open end 208 of the chamber 206 as see~ in FIG.~12 even hough the laden vehicle reaches the presele~ted level of deceleration which is suficient to ~ause the inertia weight 212 to move t~ard the c1Osed end 2~7 o~ ~he ~5 chamber 20~. This is accomplished because the pressure at the in1et openings 157, 158 attains the predetermined - s split point value before the vehicle reaches the pre- :
:'~
selected level of deeelera~iorl. When the spl~ ~ point pressure at the inlet openings 157, 158 is reached, the valve pistons ~17, 218 simultaneously move toward the , ' ' ' ,' "`,. ~.
~ 39 ~

~o~
cham~er 206 a sliyht dis-tance so that -the end e~tensions 227 thereof contac-t the inertia weight 212 which has remained in its Eirst position since the preselected level of deceleration has not been reached. The physical contact between the valve pistons 217,218 and the inertia weight 212 traps the inertia weigh-t 212 in its first position and prevents forward movement thereof at deceleration levels above the preselected deceleration level. Further movement of the valve pistons 217, 218 toward the chamber 206 will be obstructed by the inertia weight 212 thereby preventing the valve pistons 217, 218 from closing the fluid paths which provide fluid flow from the inlet openings 157, 158 to the outlet openings 161, 162 (i.e. the valve heads 223 of the valve pistons 217,218 will not engage the valve seats of the valve elements 236, 237).
This results in the pressure at the outlet openingC; ]61, 162 increasing at the same rate as the pressure at the inlet openings 157, 158 for the full ranye of inlet pressure as shown by the line passing through the points 104, 106, 108, 105 in FIG. 9. Thus, the pressure delivered to the rear bra?ce wheel cylinders 146, 147 will remain equal to the pressure delivered to the front brake wheel cylinders 143,144.
In the event of a failure (e.g. a pressure leak) in the bran~h of the brake system portion 140 which includes the conduits 149, 151, 153, the front brake wheel cylinder 143 and the rear brake wheel cylinder 147, the pressure at the inlet opening 157 of the control valve l48 due to a braking applica- -tion will be eliminated or si~nificantly reduced relative to the-pressure at the inlet opening 153 thereof.

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The}l, ~he grea-ter pressure at -the inle-t opening 158 acting on the switch actuatiny piston 177 will cause upward movement thereof in FIG. 12 to a position wherein the O-ring seal 187 is no longer in seali.ng engagement with the bore 164. This provides an unrestricted by-pass fluid path through the control valve 148 ~rom the inlet opening 158, through passage 173, counterbore 16S, bore 164, passage 199, cavity 192 and passage 204 to the outlet opening 162, Thus, the pressure delivered to the other rear brake wheel cylinder 146 through the conduit 154 will be equal to the pressure in the connected portion of the master cylinder 141.for the full range of master cylinder pressure. The upward movement of the switch actuating piston 177 moves the operating member 178 of the electrical switch 174 leftward in FIG. 12 to its circuit making position to complete the driver warniny circuit and light a driver warning lamp (not shown). Conversely, in the event of a failure ~e.g.
a pressure leak) in the branch o~ the brake system portion 140 which includes the conduits 150, 152, 154, the ~ront brake wheel cylinder 144, and the rear brake wheel cylinder 146, the . . . . . . ...... .. . ....... . . . .. . . . . . . ... . .
~0 pressure at the inlet opening 158 of the control valve 148 due : to a braking application will be eliminated or significantly : reduced relative to the pressure at the inlet opening 157. Then, the graater pressure at the inlet opening 157 acting on the switch actuating piston 177 will cause downward movement thereof ... in FIG. 12 to a posltion wherein the O-ring seal 189 is no longer in sealing engagement with the counterbore 182 of the plus 180. This provides an unrestricted by-pass fluid path through the control valve 148 from the inlet opening 157, through passage 172, sd/~ -41-,' ~. ' .

coun~er~ore 166, counterbore 182, bore 181, passages 186, counterbore 167, passage 196, cavity 191 and passage 201 to the outlet opening 161. Thus, the pressure delivered to the other rear brake wheel cylinder 147 through the conduit 153 will be equal to the pressure in the connected portion of the master cylinder 141 for the full range of master cylinder pressure. The downward movement of the switch actuating piston 177 also causes the driver warning lamp (not shown) to light as previously described.
It should be noted that the by-pass valve assembly and the anti~spike valve assembly of the control valve 18, the blend-back type of by-pass valve assembly of the control valve 18a, the stop plunger of the control valve 18b, and the warning valve assembly of the control valve 148 are optional features.
It will be understood that the invention -is not limited to including the particular type of valve means such as valve pistons 58, 58a and valve element 73. Accordingly, other types of valve means which have a movable valve member for limiting fluid flow may be used. ~t will be also understood - .. . . .... . ..... . . , . ........ . - .: . . . .. . . .. .
that the invention is not limited to including the particular type of means for disabling or obstructing movement of the valve pistons 58, 58a such as spherical inertia weight 48, and any other means which will obstruct movement of the valve pistons 58, 58a such as another piston may be used.
It is apparent that the invention is not limited to including a means for disabling or obstructing movement of the -valve pistons 58, 58a which is deceleration responsive such as inertia weight 48. Accordingly, other sd/~ -42-:

- ` ` . ........... . , -~- - ' ' "`I

l0~22 ( means such as solenoid operated rod and linkage mechan- .
isms, push-pull rod and linkage mechanisms~ and rod and linkage mechanisms actuated by vehicle positional .
changes due to loading may be employed fsr effecting :~
movement of the disabling means.
It will be understood that the claims are intended to coYer all modifications and variations of the preferred embodiments of the invention~ herein chosen for the purposes of illustration, without depart- :
~ng from ~he spirit and SCOp2 of ~he invention~
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Claims (20)

1. A valve for a vehicle hydraulic brake system of the type having a master cylinder for supplying fluid pressure to wheel brakes, said valve comprising:
a) an inlet and an outlet for transmitting fluid pressure from said master cylinder to at least one wheel brake;
b) first valving means positioned between said inlet and said outlet for transmitting the fluid pressure at said inlet to said outlet and for limiting the fluid pres-sure transmitted to said outlet relative to the pressure at said inlet when the fluid pressure at said inlet is above a predetermined level;
c) inhibiting means for preventing said first valving means from limiting the fluid pressure transmitted from said inlet to said outlet in order to maintain the outlet fluid pressure equal to the inlet fluid pressure at all levels of master cylinder operating pressure; and d) second valving means positioned between said inlet and said outlet for transmitting fluid pressure from said inlet to said outlet bypassing said first valving means.

2. The valve recited in claim 1, wherein said second valving means transmits fluid pressure from said inlet to said outlet bypassing said first valving means when a predetermined relationship exists between the pressure at said inlet and the pressure at said outlet.

3. The valve recited in claim 1, wherein said second valving means transmits fluid pressure from said inlet to said outlet bypassing said first valving means in order to increase the outlet pressure at the same rate as the inlet pressure after said first valving means establishes a predetermined differential between the inlet pressure and the outlet pressure.

4, The valve recited in claim 1, wherein said second valving means transmits fluid pressure from said inlet to said outlet bypassing said first valving means in order to increase the outlet pressure at a higher rate than the inlet pressure after said first valving means establishes a predetermined differential between the inlet pressure and the outlet pressure.

5. The valve recited in claim 1, wherein said second valving means includes a valve member normally biased into a closed position and movable into an open position for transmitting fluid pressure from said inlet to said outlet bypassing said first valving means when said first valving means establishes a predetermined relationship between the inlet pressure and the outlet pressure.

6. The valve recited in claim 19 further comprising:
a) said first valving means including a valve member movable from an open position into a closed position for limiting the pressure at said outlet relative to the fluid pressure at said inlet when the fluid pressure at said inlet reaches said predetermined level; and b) third valving means positioned between said inlet and said valve member for restricting fluid flow at said inlet for preventing movement-of said valve member toward its closed position whenever the volume of fluid flow at said inlet is above a predetermined maximum level before the pressure at said inlet reaches said predetermined level .

7. The valve recited in claim 6, wherein said third valving means includes a valve member normally urged into an open flow position and movable into a restricted flow position whenever the volume of fluid flow at said inlet is above said predetermined maximum level before the pressure at said inlet reaches said predetermined level.

8. The valve recited in claim 6, wherein:
a) said inhibiting means is movable in response to a predetermined level of vehicle deceleration from a first position for engaging said valve member into a second posi-tion where it cannot engage said valve member;
b) said inhibiting means when in said first position holds said valve member in the open position so that the pressure at said outlet will be equal to the pressure at said inlet for all levels of master cylinder operating pressure;
c) said valve member moves toward its closed position and engage and traps said inhibiting means in said first position when the pressure at said inlet reaches said predetermined level before said predetermined level of vehicle deceleration is reached; and d) said third valving means restricts fluid flow at said inlet preventing said valve member from moving toward its closed position for preventing said valve member from engaging and trapping said inhibiting means in said first position whenever the volume of fluid flow at said inlet is above said predetermined maximum level before the pressure at said inlet reaches said predetermined level.

9. The valve recited in claim 1, further: comprising:
a) said first valving means including a valve member movable between an open position and a closed position for limiting the fluid pressure transmitted to said outlet relative to the fluid pressure at said inlet when the fluid pressure at said inlet is above said predetermined level;
b) said inhibiting means being movable from a first position for engaging said valve member into a second position where it cannot engage said valve member;
c) said inhibiting means when in said first position holding said valve member in the open position so that the pressure at said outlet will be equal to the pressure at said inlet for all levels of master cylinder operating pressure; and d) obstructing means for preventing movement of said inhibiting means from said first position into said second position when the fluid pressure at said inlet is below said predetermined level.

10. The valve recited in claim 9, wherein:
a) said obstructing means includes a plunger fox engaging and preventing movement of said inhibiting means from said first position into said second position; and b) said plunger is retractable into a position where it does not prevent movement of said inhibiting means from said first position into said second position when the fluid pressure at said inlet reaches a second prede-termined level which is below said first-mentioned predetermined level.

11. A valve for a vehicle hydraulic brake system comprising:
a) an inlet for connection to a master cylinder and an outlet for connection to a wheel brake cylinder;

b) first valve means positioned between said inlet and said outlet, said first valve means having an open position for maintaining the pressure at said outlet equal to the pressure at said inlet and a closed position for limiting the pressure at said outlet relative to the pressure at said inlet;
c) means operative when the load carried by the vehicle exceeds a preselected weight level for disabling said first valve means in its open position during the entire range of inlet pressure so that the pressure at said outlet will be equal to the pressure at said inlet for all levels for inlet pressure; and d) second valve means positioned between said inlet and said outlet for transmitting fluid pressure from said inlet to said outlet bypassing said first valve means when a predetermined relationship exists between the fluid pressure at said inlet and the fluid pressure at said outlet.

12. A valve for a vehicle brake system comprising.
a) an inlet and an outlet;
b) first valve means positioned between said inlet and said outlet for transmitting fluid pressure at said inlet to said outlet, said first valve means including a valve member being movable between an open position and a closed position for limiting the fluid pressure trans-mitted to said outlet relative to the fluid pressure at said inlet;
c) disabling means being movable between a first position for engaging said valve member and a second position where it cannot engage said valve member;

d) said disabling means when in said first position holding said valve member in its open position during the entire range of inlet pressure so that the pressure at said outlet will be equal to the pressure at said inlet for all levels of inlet pressure; and e) second valve means positioned between said inlet and said outlet for transmitting fluid pressure from said inlet to-said outlet bypassing said first valve means when a predetermined relationship exists between the fluid pressure at said inlet and the fluid pressure at said outlet.

13. A valve for a vehicle brake system comprising:
a) an inlet and an outlet;
b) first valve means positioned between said inlet and said outlet for transmitting fluid pressure at said inlet to said outlet, said first valve means including a first valve member being movable between an open posi-tion and a closed position for limiting the fluid pressure transmitted to said outlet relative to the fluid pressure.
at said inlet;
c) disabling means being movable between a first position for engaging said first valve member and a second position where it cannot engage said first valve member;
d) said disabling means being in said first position fox engaging and holding said first valve member in its open position during the entire range of inlet pressure when the load carried by the vehicle is above a preselec-ted weight level so that the pressure at said outlet will be equal to the pressure at said inlet for all levels of inlet pressure;

e) said disabling means being in said second position when the load carried by the vehicle is below said pre-selected weight level whereby said first valve member will be movable between its open and closed positions so that the pressure at said outlet will be limited relative to the pressure at said inlet; and f) second valve means positioned between said inlet and said outlet, said second valve means including a second valve member movable from a closed position into an open position for transmitting fluid pressure from said inlet to said outlet bypassing said first valve means when said first valve member establishes a predetermined rela-tionship between the fluid pressure at said inlet and the fluid pressure at said outlet.

14. A valve for a vehicle brake system comprising:
a) an inlet and an outlet;
b) first valve means positioned between said inlet and said outlet for transmitting fluid pressure at said inlet to said outlet, said first valve means including a first valve member being movable between an open position and a closed position for limiting the fluid pressure transmitted to said outlet relative to the fluid pressure at said inlet when the fluid pressure at said inlet is above a first predetermined level;
c) disabling means being movable in response to a preselected level of vehicle deceleration from a first position for engaging said first valve member to a second position where it cannot engage said first valve member;
d) said disabling means remaining in said first posi-tion for engaging and holding said first valve member in its open position when the pressure at said inlet reaches said first predetermined level before said preselected level of vehicle deceleration is reached so that the pressure at said outlet will be equal to the pressure at said inlet;
e) said disabling means moving into said second posi-tion when said preselected level of vehicle deceleration is reached before the pressure at said inlet reaches said first predetermined level whereby said first valve member will be movable between its open and closed positions so that the pressure at said outlet will be limited relative to the pressure at said inlet when the pressure at said inlet exceeds said first predetermined level; and f) second valve means positioned between said inlet and said outlet, said second valve means including a second valve member movable from a closed position into an open position for transmitting fluid pressure from said inlet to said outlet bypassing said first valve means when said first valve member establishes a predetermined rela-tionship between the fluid pressure at said inlet and the fluid pressure at said outlet.

15. A valve for a vehicle dual brake system of the type including a master cylinder having first and second separated portions for supplying fluid pressure to first and second separated wheel brakes, respectively, through first and second separated fluid branches, respectively, said valve comprising:
a) first and second inlets for connection to said first and second master cylinder separated portions, respectively;
b) first and second outlets for connection to said first and second separated wheel brakes, respectively;
c) first valve means disposed in said first branch for transmitting fluid pressure at said first inlet to said first outlet and for limiting the fluid pressure transmitted to said first outlet relative to the fluid pressure at said first inlet when the fluid pressure at said first inlet is above a predetermined level;
d) second valve means, corresponding to said first valve means, disposed in said second branch for transmit-ting fluid pressure at said second inlet to said second outlet and for limiting the fluid pressure transmitted to said second outlet relative to the fluid pressure at said second inlet when the fluid pressure at said second inlet is above said predetermined level; and e) inhibiting means for simultaneously preventing said first and second valve means from limiting the fluid pressure transmitted from said first and second inlets to said first and second outlets, respectively, in order to maintain the pressure at said first and second outlets equal to the pressure at said first and second inlets, respectively, at all levels of pressure in said first and second master cylinder portions, respectively.

16. The valve recited in claim 15, wherein:
a) said first valve means includes a first valve member movable between an open position and a closed position for limiting the pressure transmitted to said first outlet relative to the pressure at said first inlet when the pressure at said first inlet is above said predetermined level;
b) said second valve means includes a second valve member, corresponding to said first valve member, movable between an open position and a closed position for limit-ing the pressure transmitted to said second outlet rela-tive to the pressure at said second inlet when the pres-sure at said second inlet is above said is above said predetermined level; and c) said inhibiting means is operative for simultan-eously holding said first and second valve members in their open positions so that the pressures at said first and second outlets will be equal to the pressures at said first and second inlets, respectively, for all levels of pressure in said first and second master cylinder portions, respectively, when the load carried by the vehicle exceeds a predetermined weight level.

17. The valve recited in claim 15, wherein:
a) said first valve means includes a first valve member movable between an open position and a closed po-sition for limiting the pressure transmitted to said first outlet relative to the pressure at said first inlet when the pressure at said first inlet is above said predeter-mined level;
b) said second valve means includes a second valve member, corresponding to said first valve member, movable between an open position and a closed position for limit-ing the pressure transmitted to said second outlet relative to the pressure at said second inlet when the pressure at said second inlet is above said predetermined level;
c) said inhibiting means is movable between a first position for simultaneously engaging said first and second valve members and a second position where it cannot engage said first and second valve members; and d) said inhibiting means when in said first position simultaneously engaging and holding said first and second valve members in their open positions so that the pres-sures at said first and second outlets will be equal to the pressures at said first and second inlets, respec-tively, for all levels of pressure in said first and second master cylinder portions, respectively.

18. The valve recited in claim 17, wherein;
a) said inhibiting means moves into said second position when a predetermined level of vehicle decelera-tion is reached before the pressures at said first and second inlets reach said predetermined level; and b) said inhibiting means remains in said first posi-tion when the pressures at said first and second inlets reach said predetermined level before said predetermined level of vehicle deceleration is reached.

19. The valve recited in claim 18, wherein said first and second valve members move toward their closed posi-tions and engage and trap said inhibiting means in said first position when the pressures at said first and second inlets reach said predetermined level before said prede-termined level of vehicle deceleration is reached.

20. The valve recited in claim 15, further comprising:
a) third valve means positioned between said first inlet and said first valve means in said first branch for transmitting fluid pressure from said first inlet to said first outlet bypassing said first valve means upon a failure in said second branch; and b) fourth valve means positioned between said second inlet and said second valve means in said second branch for transmitting fluid pressure from said second inlet to said second outlet bypassing said second valve means upon a failure in said first branch.