GB1565208A

GB1565208A - Vehicle braking systems

Info

Publication number: GB1565208A
Application number: GB39882/75A
Authority: GB
Inventors: David J Knight; Sidney A Lippiatt; Anthony Harris
Original assignee: Bendix Westinghouse Ltd
Current assignee: Honeywell UK Ltd
Priority date: 1976-01-02
Filing date: 1976-09-22
Publication date: 1980-04-16
Also published as: DE2643805A1; SE431184B; ATA715576A; DE2643805B2; DE2643805C3; SE7610774L; IT1071610B; BE846641A; FR2336284B1; NL181639C; AT377948B; FR2336284A1; NL7610308A

Description

(54) VEHICLE BRAKING SYSTEMS (71) We, BENDIX WESTINGHOUSE LIMITED, a Company incorporated under the laws of Great Britain, of Douglas Road, Kingswood, Bristol, England, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed.

to be particularly described in and by the following statement: This invention relates to vehicle braking systems for use in tractor/trailer combinations wherein a signal is provided on a tractor vehicle for controlling the brakes on the tractor vehicle and also on a trailer vehicle when coupled thereto. The invention is especially concerned moreover with such systems wherein a load sensing device is provided for modulating the signal in respect of control of braking to rear wheels of the tractor.

According to the present invention there is provided a vehicle fluid pressure braking system for use in a tractor and trailer vehicle combination including a relay valve having a pressure responsive assembly which is operable in response to a braking circuit pressure on the tractor to operate a valve element to apply an output pressure to a brake control line for the trailer, the system including a variable load valve which is connected such as to produce a modified rear braking circuit pressure for reduced tractor rear wheel braking with reduced loading of the tractor and the pressure responsive assembly of said relay valve including pressure responsive means which is responsive also to the output of the variable load valve in a sense to increase the resultant trailer braking in relation to the tractor rear wheel braking with reduction of load on the tractor.

The relay valve may comprise a selflapping valve having control input, output and exhaust ports, a main pressure responsive member subject on one side to control port pressure and subject on the other side to an output pressure and a further pressure responsive member co-operating with the main pressure responsive member in controlling the self-lapping valve and operating to assist the control port pressure in dependence upon a difference between the control port pressure and a load dependent pressure controlling tractor brake actuators.

In order that the invention may be more clearly understood and readily carried into effect. the same will be further described by way of example with reference to the accompanying drawings of which Figure 1, illustrates a fluid pressure amplifier device for use in a system according to the present invention, Figure 2. illustrates a vehicle braking system for a towing truck or tractor vehicle adapted to accept the attachment of a trailer by way of a draw-bar, Figure 3, illustrates a modification of the system shown in Figure 2 suitable for a tractor and semi-trailer combination in which it may be regarded as acceptable to couple a tractor vehicle and a semi-trailer in articulated fashion with the braking system of the combination arranged as a split dual system with one service reservoir feeding the tractor service brake circuit and the other the semi-trailers service and emergen cv lines, or (the other the semi-trailer service brake).

Figure 4. illustrates a preferred form of amplifier valve, Figure 5. illustrates a simplified form of the amplifier valve of Figure 4, Figure 6. illustrates a modification of the system circuit for use in the system of Figure 1. Figure 2. or Figure 3, Figure 7, illustrates a further modified form of the amplifier valve of Figure 4 including a check valve and Figure 8, illustrates a self-lapping valve suitable for use as an alternative to the check valve of Figure 7.

Referring to Figure 1, the fluid pressure relay valve which is shown therein consists of a self-lapping relay valve arrangement comprising an input port 1, in a valve body portion 2, an output port 3 and an exhaust port 4. The valve arrangement comprises a tubular valve member 5 which is sealingly slidable in the valve body portion 2 and is spring loaded upwards by a spring 6 towards an inlet valve seat 7 and presenting to an exhaust valve member 8 an upwardly facing exhaust valve seat. The exhaust valve member 8 is carried by a piston 9 which is sealingly slidable in a cylindrical chamber of a further body member 10 mounted above the valve body member 2 as shown.The piston 9 is sealingly slidable within the cylindrical chamber and has an upwardly extending rod 11 which sealingly passes through the upper wall of the cylinder into a further chamber 12 wherein it abuts a further piston 13 forming a further pressure responsive member. The upper opening of the body 10 is closed by a closure portion 14.

The fluid pressure amplifier device in addition to the afore-mentioned input. output and exhaust ports, has three control ports which, for convenience. are labelled A, B and C. as shown. Ports A and B communicate respectively with the upper and lower sides of the further pressure responsive member 13 and port C communicates with the upper side of the main pressure responsive member 9. the underside of which is subject to the output pressure of the valve and also to the thrust of a spring 15 which counter-balances the weight of the pressure responsive members and tends to maintain the valve member 8 in a position as shown with the inlet valve closed.

Also shown in Figure 1 are diagrammatical representations of the sources of control pressures which are presented at the input ports A. B and C. the input port A receives a pressure from a double-check valve 17. the respective inputs of which correspond to the front and rear braking pressures for the tractor vehicle and are denoted F and R respectively. The output of the doublecheck valve 17 is also applied as an input to a further double-check valve 18 the output of which appears at the input port C and the second input of which, denoted S. corresponds to the tractor secondary braking pressure.The input port B derives its input which is denoted as Rm from the output of a variable load valve which modulates the pressure to the rear brake actuators of the tractor vehicle in accordance with the loading of the respective rear axle to reduce the rear wheel braking with reduced loads on the tractor vehicle.

Considering now the operation of the valve of Figure 1, the output appearing at the port 3 is applied to the trailer service brake coupling and the input at the port 1 is derived from a reservoir on the tractor which supplies the trailer line. As will be seen from Figures 2 and 3, (in each of which the supply to the relay valve is shown dotted) depending upon the arrangement of the system, this may be a separate reservoir for supplying the trailer or the pressure may be derived from one of the reservoirs supplying a dual circuit valve for the tractor service brakes which produces pressures F and R.

Assuming that the tractor vehicle is fully laden. the variable load valve on the tractor causes the pressure at the control port B in the case of a service braking application. to be substantially equal to the service braking pressure derived via the double-check valve 17 and applied to the input port A. In addition, the output from the double-check valve 17 appears via the double-check valve 18 at the input port C since the secondary brake is assumed not applied. Under these circumstances the pressures across the pressure responsive member 13 are substantially mutually cancelling and the output pressure derived from the self-lapping valve arrangement is substantially equal therefore to the only controlling pressure which appears at the port C.

In the case of a secondary braking application. and assuming that the vehicle is loaded. the secondary braking pressure is applied via the double-check valve 18 to the port C and the valve again operates as a normal self-lapping relay valve in response to this presure at the port C.

Again. in the case of a service application with the rear axle of the tractor appreciably less than fullv loaded. the pressure at the port B as derived from the variable load valve of the vehicle, will be substantially less than the service pressure at the port A with the result that the difference between these two pressures acts such as to assist a service pressure at the port C such that the output pressure derived from the self-lapping valve is appropriately higher than the service braking pressure at the port C. With a lightly loaded tractor therefore, there is a substantially relatively higher degree of braking produced in the trailer to tend to reduce the possibilities of the trailer causing the tractor to lose control by virtue of a jack-knifing or similar effect.

A secondary braking application in the absence of a service application in the part-loaded condition of the tractor again renders the self-lapping valve operable as a normal self-lapping relay valve in response to the secondary braking pressure applied at the input port C via the double-check valve 18.

In the event of a failure of the front braking circuit of the tractor, it will be appreciated that, except for the front tractor brake failure, the system continues to operate to give tractor rear brakes and trailer braking in accordance with the pressures at ports A and B. If, on the other hand, the rear braking circuit fails, the front brake circuit of the trailer provides a pressure via the double-check valve 17 at the input port A and at the input port C via the doublecheck valve 18. In this event, the maximum possible differential between the front brake pressures and the trailer brake pressures will be achieved by virtue of the loss of an upward pressure acting on the pressure responsive member 13 via the input port B.

The result therefore, in either case. is a tendency for the trailer braking to maintain the tractor/trailer combination in a controllable condition.

Referring now to Figure '. this shows in diagrammatical form components of a tractor braking system incorporating a fluid pressure amplifier device as shown in Figure 1. The system includes two separate service brake circuit reservoirs denoted references 21 and 22, a trailer brake pressure source reservoir 23 and a secondary/park reservoir 24. The service reservoirs feed the inputs to a dual-circuit foot valve 25 which supplies as shown, the front wheel brake and rear wheel brake service portions of spring brake actuators on the tractor. These are supplied via quick release valve S 26 and 27 respectively, the pressure to the rear actuators being conti olled by a variable load valve 28 which is dependent upon the loading of the rear axle.In addition to supplying the front and rear service portions of the tractor actuators, the dual foot valve supplies the pressures F and R to the double-check valve 17, and the output of this is applied to the above mentioned input port A of the fluid pressure amplifier device 29 which is substantially as described above with reference to Figure 1. The double-check valve 18. referred to above. derives one input pressure from 17 and its other input pressure from a secondary braking/parking brake control valve denoted by reference 30, supplied from the secondary/park reservoir 24. An inverse controlling pressure to the spring brake portions of the front and rear actuators is also derived from 30 via differential protection valves 32 and 33.The amplifier device thus provides a service output pressure to the trailer brakes via a service line coupling 34. The source of the pressure at 34 is the reservoir 23 which also supplies a relay valve 35 and which has a controlling input from the secondary/park reservoir 24. The relay valve 35 provides the emergency line pressure at the emergency line coupling 36 via a supply dump valve 37 which senses a difference between the service line pressure 34 and the foot valve output pressure controlling it to vent the emergency line pressure. The operation of a supply dump valve is described in greater detail in Applicants' [Technical Pamphlet No. 9/010.1 In the modification for a semi-trailer of the arrangement of Figure 2 as shown in Figure 3. the dual foot valve 45 is again supplied from two reservoirs 41 and 42.The foot valve circuit supplied by 42 now only controls the trailer brakes in the semi-trailer via the relay valve 49. the supply to which is shown chain-dotted. The trailer is assumed to be coupled to the tractor in articulated fashion. The secondary park circuit is substantially as shown in Figure 2, except that in the present case. the relay emergency valve is supplied from the same reservoir 42 as that which supplies the trailer service circuit controlled bv one side of the dual foot valve 45.

The fluid pressure amplifier device 49 has inputs A and C in Figure 3 derived via the associated double-check valves denoted by references 50 and 51. The double-check valve 50 now receives a tractor braking signal pressure on one side and a trailer braking signal pressure on the other side which is also applied to one input of the double-check valve 51. The other input of 51 is derived from the secondary brake output from the secondary brake control valve 53. A variable load valve 48 is again included in the service braking circuit in a position such as to be effective upon the tractor rear brakes but not on the tractor front brakes and also to provide a means for. in addition modulating the signal which is applied to the input B of the pressure amplifier device 49. A supply dump valve 57 is also provided as in Figure 2.

From the foregoing, it will be appreciated that the system illustrated in Figure 3 operates on similar principles to that described for the arrangement of Figures 1 and 2. such that in less than fully laden conditions of the tractor. the trailer braking is arranged to be modulated in a sense that the fluid pressure for the trailer in relation to the rear tractor brakes is the more. the less is the loading of the rear tractor axle.

In Figure 4 there is shown an amplifier device which is of generallv similar construction to that outlined in Figure 1. but it will be observed that the output from the vari able load valve is now connected to a discrete pressure responsive area 92 of the main piston 91. Separate input ports for pressures AF and AR are now provided connected to individual pressure input relay chambers of the device provided on opposite sides of a separate piston 95. These receive front and rear tractor brake pressures respectively and ports B and C receive the load modulated tractor rear brake pressure and secondary brake pressure respectively. This enables the check valves employed in the said Figure 1 to be dispensed with.To provide a separate input area 93 for secondary trailer brake pressure. a further piston 96 is also provided which is subject to the output pressure of the relay valve. It will also be observed that the relative areas with which the A, B and C input ports communicate are inter-changed whereby the influence of the load modulated pressure at port B is of proportionately greater effect in counteracting whichever is the larger of the pressures at port AR or AF.

Referring to Fig. 5. the amplifier device shown therein is a simplified version of that shown in Figure 4. wherein the C input is blanked-off and the lower piston 96 omitted so that 92 becomes the relay valve reaction area. This simplified device may be employed in a system according to the invention where secondary braking is provided from one half of the dual service braking portion of the system. In such a system a hand valve would be provided for a parking brake only.

Referring to Figure 6. this illustrates a dual foot valve denoted by the reference 71 which in accordance with the effort exerted thereon bv a vehicle driver, provides output pressures to the front brakes at the line F and the rear brakes at the line R, the apparatus shown being part of the tractor vehicle system. The output F is fed as one control input to a multi-input relay valve 72 also supplied bv the rear foot brake circuit pressure and further controlled by a pressure Rm derived via the variable load valve 73 which senses the loading on one rear axle of the vehicle.The output of the variable load valve 73 feeds the rear service brakes of the tractor in accordance with the vehicle loading and the input Rm fed to the relay valve 72 is derived in accordance with one embodiment of the present invention via a check valve 74 interposed between the rear brake lines and the amplifier valve. A further check valve 75 is provided between the input Rm to the amplifier valve and the line carrying pressure R to the amplifier valve derived from the dual foot valve.

In operation of the arrangement shown in Figure 6. it will first be appreciated that this is a simplification of a system such as illustrated in Figure 2 and differs therefrom in its essentials only by the inclusion of the check valves 74 and 75.

When a brake application is made, air under pressure is delivered to the relay valve 72 and the variable load valve 73. The output from the variable load valve is fed to the rear brakes and also as a controlling input Rm to the amplifier valve 72. As the vehicle responds to the braking, a dynamic load transfer occurs from the rear axle to the forward axles and the variable load valve 73 tends to deliver a lower pressure than that which existed at the commencement of the brake application. By virtue however of the check valve 74 interposed between the variable load valve and the controlling input for the pressure Rm to the amplifier valve 72. the pressure applied to the controlling input Rm to the amplifier valve 72 cannot follow the output of the variable load valve 73.Accordingly, the value of the controlling input derived from the variable load valve remains at the maximum value which is established during the braking application.

When the brake application is released. the foot valve 71 connects its output lines F and R to atmosphere to vent the brakes and the entrapped pressure Rm between the check valve 74 and the amplifier valve 72 is released via the check valve 75 into the rear brake control line R.

Bv virtue of the arrangement of Figure 6, it will be seen that a tendency for the trailer brakes to increase as a result of dvnamic load shift from the rear axles of the tractor vehicle may be prevented in a system such as shown in Figure 2.

Referring to Figure 7 of the drawings, this shows a self-lapping relay valve for use in a tractor/trailer braking system such as described in Figure 2 or Figure 3. The relay valve is substantially similar in most respects to the relay valve shown in Figure 4, but with an exception that the output port pressure does not communicate directly with the underside of the main pressure responsive member. Instead, a dividing wall is formed between the output port pressure and the region beneath the main pressure responsive member and communication between the region at output port pressure and the underside of the main pressure responsive member, is provided via a biassed check valve 62.

When a brake application is made, the inlet 'exhaust valve assembly of the relay valve opens the air in passed from the inlet to the outlet. In a valve such as described in Figure 4. the pressure being passed would act in an upward direction on the main pressure responsive member formed by the central lower piston to give a reaction and cause the valve to move towards a lapped condition in which the inlet and exhaust valves are both closed, the output pressure balancing the downward force exerted by the various pressure responsive members acting in accordance with the input pressure applied thereto as in Figure 2 or Figure 3.

However, in the present arangement, the output pressure is channelled through a passage 61 to the underside of the springloaded check valve 62. When the pressure reaches the pressure at which the check valve is set, which is determined by the spring 63 and the adjustment of the adjusting screw 64, the check valve opens and allows air under pressure to pass via the opening 65 to the underside of the main pressure responsive member. For a short time at the beginning of an application there is therefore no upward reaction on the piston stack and there is a corresponding increase in delivery pressure as applied to the trailer service braking.

When the tractor rear brake pressure is reduced after a brake application, the check valve is closed under the influence of spring 63 but a passage for descending pressure is provided via passage 65, one-way valved passages 66 in the member 62 and passage 61. It will be understood therefore that the selectable bias provided by the check valve is not effective during falling brake pressures.

The advantage of an arrangement such as described above when used to replace the valve of Figure 4, is that the characteristic of the braking pressure applied to the trailer as compared with the service braking pressure, is adjustable for rising brake pressures by selection of the adjustment of the screw 64 thereby building in a selectable bias on the pressure responsive members as a whole and enabling the characteristic to be positioned within a required band such as may be specified for any particular equipment installation.

If it is required to provide an adjustable bias such as referred to and that the bias shall be effective for both rising and falling brake pressures, the check valve employing parts 62, 63 and 64 of Figure 7 may be replaced by a self-lapping valve device. One form of self-lapping valve device which may be employed for this purpose is shown in Figure 8.

The self-lapping valve of Figure 8 comprises a generally cylindrical housing 81 with one open end closed by a cap 82 having an exhaust port. The housing 81 may be adapted as a bolt-on attachment to a relay valve such as that of Figure 7, to provide connections therewith to passages 61 and 62 as labelled in Figure 7 and Figure 8. Within the housing 81 there is a sealingly slidable piston 83 spring loaded by a spring 84 between 83 and the cap 82. Piston 83 carries an inlet valve member 85 biassed towards a closed position by a light spring 86. the exhaust valve seat 87 of the self-lapping valve being carried by a tubular screw adjustable member 88 in the end cap 82.

With the valve of Figure 8 added as a replacement for the check valve of Figure 7, the arrangement operates in a similar manner for rising pressure at passage 61. Thus rising pressure at 61 increases by an amount determined by the compression force required to compress spring 84 to such an extent that the exhaust seat 87 is closed by valve member 85 being carried into engagement therewith bv piston 83. The inlet valve then unseats and provides connection between passages 61 and 65 to apply pressure under the balancing piston of the amplifier valve (Figure 7). The pressure is passage 65 and under the balancing piston then follows that in passage 61 but is less than the pressure 61 by an amount dependent upon the adjustment of the screw adjustment of 88.Furthermore. by virtue of the selflapping nature of the valve of Figure 8, this continues to be so for subsequent reductions of pressure in passage 61 on reduction of braking pressure in the tractor braking system. down to the value of pressure at which valve member 85 unseats from valve seat 87.

It will be appreciated bv skilled persons that by suitable design of the relay valve itself. it may be provided with a suitable flange or surface whereby the check valve is shown in the valve arrangement of Figure 7 or the self-lapping valve of Figure 8. can be included or not. as may be required in a particular system. If such a check valve or self-lapping valve is not required in a particular system, the flange or surface on the relay valve may simply be covered by a suitable cover plate affording a direct connection between passages 61 and 65.

A check valve or self-lapping valve of the type such as shown in Figure 7 or Figure 8, respectively may be employed in any selflapping relay valve as employed in any system in accordance with the present invention.

WHAT WE CLAIM IS: 1. A vehicle fluid pressure braking system for use in a tractor and trailer vehicle combination including a relay valve having a pressure-responsive assembly which is operable in response to a braking circuit pressure on the tractor to operate a valve element to apply an output pressure to a brake control line for the trailer. the system including a variable load valve which is connected such as to produce a modified rear braking circuit pressure for reduced tractor rear wheel braking with reduced loading of the tractor and the pressure-responsive assembly of said relay valve including pressureresponsive means which is responsive also to the output of the variable load valve in a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. balancing the downward force exerted by the various pressure responsive members acting in accordance with the input pressure applied thereto as in Figure 2 or Figure 3. However, in the present arangement, the output pressure is channelled through a passage 61 to the underside of the springloaded check valve 62. When the pressure reaches the pressure at which the check valve is set, which is determined by the spring 63 and the adjustment of the adjusting screw 64, the check valve opens and allows air under pressure to pass via the opening 65 to the underside of the main pressure responsive member. For a short time at the beginning of an application there is therefore no upward reaction on the piston stack and there is a corresponding increase in delivery pressure as applied to the trailer service braking. When the tractor rear brake pressure is reduced after a brake application, the check valve is closed under the influence of spring 63 but a passage for descending pressure is provided via passage 65, one-way valved passages 66 in the member 62 and passage 61. It will be understood therefore that the selectable bias provided by the check valve is not effective during falling brake pressures. The advantage of an arrangement such as described above when used to replace the valve of Figure 4, is that the characteristic of the braking pressure applied to the trailer as compared with the service braking pressure, is adjustable for rising brake pressures by selection of the adjustment of the screw 64 thereby building in a selectable bias on the pressure responsive members as a whole and enabling the characteristic to be positioned within a required band such as may be specified for any particular equipment installation. If it is required to provide an adjustable bias such as referred to and that the bias shall be effective for both rising and falling brake pressures, the check valve employing parts 62, 63 and 64 of Figure 7 may be replaced by a self-lapping valve device. One form of self-lapping valve device which may be employed for this purpose is shown in Figure 8. The self-lapping valve of Figure 8 comprises a generally cylindrical housing 81 with one open end closed by a cap 82 having an exhaust port. The housing 81 may be adapted as a bolt-on attachment to a relay valve such as that of Figure 7, to provide connections therewith to passages 61 and 62 as labelled in Figure 7 and Figure 8. Within the housing 81 there is a sealingly slidable piston 83 spring loaded by a spring 84 between 83 and the cap 82. Piston 83 carries an inlet valve member 85 biassed towards a closed position by a light spring 86. the exhaust valve seat 87 of the self-lapping valve being carried by a tubular screw adjustable member 88 in the end cap 82. With the valve of Figure 8 added as a replacement for the check valve of Figure 7, the arrangement operates in a similar manner for rising pressure at passage 61. Thus rising pressure at 61 increases by an amount determined by the compression force required to compress spring 84 to such an extent that the exhaust seat 87 is closed by valve member 85 being carried into engagement therewith bv piston 83. The inlet valve then unseats and provides connection between passages 61 and 65 to apply pressure under the balancing piston of the amplifier valve (Figure 7). The pressure is passage 65 and under the balancing piston then follows that in passage 61 but is less than the pressure 61 by an amount dependent upon the adjustment of the screw adjustment of 88.Furthermore. by virtue of the selflapping nature of the valve of Figure 8, this continues to be so for subsequent reductions of pressure in passage 61 on reduction of braking pressure in the tractor braking system. down to the value of pressure at which valve member 85 unseats from valve seat 87. It will be appreciated bv skilled persons that by suitable design of the relay valve itself. it may be provided with a suitable flange or surface whereby the check valve is shown in the valve arrangement of Figure 7 or the self-lapping valve of Figure 8. can be included or not. as may be required in a particular system. If such a check valve or self-lapping valve is not required in a particular system, the flange or surface on the relay valve may simply be covered by a suitable cover plate affording a direct connection between passages 61 and 65. A check valve or self-lapping valve of the type such as shown in Figure 7 or Figure 8, respectively may be employed in any selflapping relay valve as employed in any system in accordance with the present invention. WHAT WE CLAIM IS:

1. A vehicle fluid pressure braking system for use in a tractor and trailer vehicle combination including a relay valve having a pressure-responsive assembly which is operable in response to a braking circuit pressure on the tractor to operate a valve element to apply an output pressure to a brake control line for the trailer. the system including a variable load valve which is connected such as to produce a modified rear braking circuit pressure for reduced tractor rear wheel braking with reduced loading of the tractor and the pressure-responsive assembly of said relay valve including pressureresponsive means which is responsive also to the output of the variable load valve in a

sense to increase the resultant trailer braking in relation to the tractor rear wheel braking with reduction of load on the tractor.

2. A vehicle fluid pressure braking system as claimed in Claim 1, said pressureresponsive means comprising an additional piston or assembly of piston areas subject on opposite sides to input and output pressures respectively of the variable load valve.

3. A vehicle fluid pressure braking system as claimed in Claim 2. wherein the additional piston or assembly of piston areas is subject on an upper side to the tractor vehicle rear braking circuit pressure and on a lower side to the modified rear braking circuit pressure such as to exert a downward force on the valve element.

4. A system as claimed in Claim 1.

characterised by the system including means whereby said braking pressure on the tractor to which the relay valve is responsive is the more predominant of tractor front and rear brake circuit pressures.

5. A system as claimed in Claim 1, characterised bv the system including means whereby said braking pressure on the tractor to which the relay valve is responsive is the more predominant of front. rear or secondary brake circuit pressures.

6. A system as claimed in Claim 4 or 5, said last mentioned means comprising check valve means.

7. A system as claimed in Claim 4 or 5, said last mentioned means comprising a non-compounding assembly of piston areas.

8. A vehicle braking system as claimed in any preceding claim. the relay valve being a self-lapping valve in which the output trailer control line pressure is applied to a balancing chamber to effect self-lapping. via an additional valve means being disposed in the communication between the relay output and the balancing chamber to impose a predetermined bias whereby the tractor brake pressure is subject to a prescribed trailer control line pressure predominance.

9. A vehicle braking system as claimed in Claim 8. said additional valve means comprising a biassed check valve.

10. A vehicle braking system as claimed in Claim 8. said additional valve means comprising a self-lapping valve.

11. A vehicle braking system as claimed in any preceding claim. wherein means is provided for storing the output of the variable load valve as applied to the relav valve to tend to make it independent of subsequent dvnamic variations of axle loading during a brake application.

12. A system as claimed in Claim 11. the last mentioned means comprising a check valve which enables maximum attained value of the said input signal to be entrapped and a path being provided for release thereof on subsequent tractor brake release.

13. A braking system substantially as described herein with reference to Figure 1, or Figure 1 as modified in accordance with one or more of the valves described herein with reference to Figures 2 to 8 of the accompanying drawings.