GB1592904A

GB1592904A - Vehicle braking system

Info

Publication number: GB1592904A
Application number: GB3783877A
Authority: GB
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1977-09-10
Filing date: 1977-09-10
Publication date: 1981-07-08
Also published as: IN150975B; JPS5464883U

Description

(54) VEHICLE BRAKING SYSTEM (71) We, AUTOMATIVE PRO DUCTS LIMITED, a British company of Tachbrook Road, Leamington Spa, Warwickshire, CV31 3ER, 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 he following statement: The invention relates to a vehicle braking system incorporating a hydraulic master cylinder.

In those braking systems which include disc brakes and drum brakes it is known to fit a hold-off or delay valve into the brake line between the master cylinder and the disc brakes to prevent the disc brakes from producing effective braking effort until there is sufficient pressure at the drum brakes to overcome the effect of the usual shoe pull-off springs. This is to save wear of the front brake pads. The hold-off valve is necessarily an additional expense and requires additional labour time to fit to a new vehicle.

Variable ratio master cylinders have been proposed for use in vehicle braking systems as a method of achieving a good braking effort without an excessive brake pedal travel. A stepped piston is used to provide a large effective piston area and a small effective piston area. Initial movement of the master cylinder piston, which takes up the clearances in the brake themselves, utilises the large piston area and further movement to apply the brakes firmly utilises the small piston area. A metering valve smoothes the effect of the transition from the hydraulic ratio ob- tained with the large piston area to that obained with the small piston area.

A disadvantage of a variable ratio master cylinder is that usually the change in hydraulic ratio is noticeable to the driver despite the use of a metering valve.

An objective of the present invention is to provide a master cylinder for a hydraulic brake system which has disc brakes and drum brakes which will combine advantages of known hold-off valves and known variable ratio master cylinders.

The present invention provides a vehicle braking system having drum brakes, disc brakes, and a vehicle brake master cylinder comprising first and second pistons acting to pressurise first and second chambers respectively, one chamber controlling the disc brakes and the other chamber controlling the drum brakes, the master cylinder being such that the force transmitted to each piston is directly proportional to the pedal effort applied by the driver throughout the operational range, and wherein the first piston has two effective areas for pressurising the first chamber the larger area being effective during initial operation of the master cylinder and the smaller area being effective during subsequent operation.

Advantageously, the two pistons are mounted in-line within a common housing.

In this case, the driver-controlled effort acts on one of the pistons and the master cylinder is so dimensioned that the force transmitted via the hydraulic fluid to the other piston is directly proportional to the pedal effort. Said one piston may be either the first piston or the second piston.

The first chamber may be provided with an inlet port connectible to a tank of hydraulic fluid and an outlet port connected to the disc brakes.

Preferably, the first piston is provided with a metering valve for smoothing out the pressure difference in the first chamber when the effective area of the first piston changes from the larger value to the smaller value. In this case, the metering valve starts to open at a first, low value of the pressure of the first chamber and is fully open at a second high value of the pressure of the first chamber.

The master cylinder may further comprise a control chamber, movement of the first piston in the brake-applying direction varying the volume of the control chamber. The master cylinder may also further comprise a first nonreturn valve allowing fluid flow from the control chamber to the first chamber, and a second non-return valve allowing fluid flow from the inlet port to the control chamber.

Advantageously, the metering valve has a metering valve member responsive to the pressure in the first chamber and the pressure in the control chamber, the metering valve member controlling the control chamber pressure. Preferably, the metering valve member comprises a plunger slidable in a bore in the first piston and biassed towards the closed position of the metering valve by resilient means. The plunger may be provided with an axial bore which is normally closed by a closure member, but which is opened by restraining movement of the closure member relative to the plunger in the direction opposing the loading of said resilient means to allow hydraulic fluid to enter or leave the control chamber.

The volume of the control chamber may decrease with brake-applying movement of the first piston, in which case the metering valve is so arranged that the pressure in the first chamber and in the control chamber both act on the plunger in opposition to said resilient means, whereby, until the pressure in the first chamber reaches said, first, low value, the pressure in both the first chamber and the control chamber increases at the same rate with hydraulic fluid passing from the control chamber to the first chamber through the first non-return valve, and, when the metering valve becomes operative at said first, low value, hydraulic fluid passes from the control chamber to the inlet port through the metering valve, the pressure in the control chamber decreasing with increasing pressure in the first chamber until control chamber pressure reaches tank pressure at said second, high value of the pressure in the first chamber.

Alternatively, the volume of the control chamber may increase with brake-applying movement of the first piston, in which case the metering valve is so arranged that the pressure in the first chamber acts on the plunger in opposition to control chamber pressure and said resilient means, whereby, until the pressure in the first chamber reaches said first, low value, the control chamber pressure remains at tank pressure by the passage of hydraulic fluid from the inlet port through the second non-return valve, and, when the metering valve becomes operative at said first low value, hydraulic fluid passes from the first chamber to the control chamber through the metering valve, the control chamber pressure increasing with increasing pressure in the first chamber until control chamber pressure equals the pressure in the first chamber at said second, high value.

Preferably, the second chamber is provided with an inlet port connectible to a tank of hydraulic fluid and an outlet port connected to the drum brakes.

When said one piston is the first piston, conveniently one piston area of the second piston is subject to the pressure in the first chamber to create a brake-applying force on the second piston, and another piston area of the second piston is subject to pressure in the control chamber to modify the brake applying force on the second piston created by the pressure in the first chamber, the arrangement being such that hydraulic fluid is displaced from the first chamber through the associated outlet port at a high rate with respect to the brake-applying movement of the first piston relative to the second piston until, when the metering valve becomes operative at said first, low value, said displacement of hydraulic fluid is reduced to a low rate, the control chamber pressure acting on the first piston to increase progressively the ratio of pressure in the first chamber to the drivercontrolled pedal effort on the first piston as the pressure in the first chamber increases from said first, low value to said second, high value, and being such that the effect of the control chamber pressure acting on said another piston area of the second piston is to maintain the pressure in the second chamber substantially proportional to said pedal effort.

In this case, where the volume of the control chamber decreases with brake-applying movement of the first piston, said another piston area of the second piston may be so arranged that the controls chamber pressure acts on the second piston to augment the brake-applying force created by the pressure in the first chamber acting on said one piston area. Here, the first and second pistons may be telescoped together so that one end wall of the control chamber is formed by the second piston.

Alternatively, where the volume of the control chamber increases with brake-applying movement of the first piston, said another piston area of the second piston may be so arranged that the control chamber pressure acts on the second piston to counteract the brake-applying force created by the pressure in the first chamber acting on said one piston area.

Where said one piston is the second piston, the first piston may be stepped and have a portion acting in the same bore as that in which the second piston acts, whereby the force transmitted to the first piston via the hydraulic fluid is directly proportioned to the pedal effort.

Preferably, the braking system further comprises an indicator, the indicator comprising a stepped plunger having a first piston area for connection to the chamber associated with said one piston, a second piston area for connection to the chamber associated with said other piston, and a third piston area for connection to the control chamber, the arrangement being such that the pressures in said chambers act on the stepped plunger to produce forces on the stepped plunger which are in the same proportional relationship to each other as the corresponding forces on the other piston produced by the same pressures, the forces on the stepped plunger being in the same direc tions relative to each other as the corresponding forces on the other piston so that the forces are normally balanced, any failure in the braking system resulting in the pressure in the chambers associated with either of the pistons being lower than normal, causing movement of the plunger to operate a warning device.

Three embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram of the first embodiment showing a master cylinder and failure indicator in cross-section; Figure 2 is a diagram, similar to Figure 1, of the second embodiment; Figure 3 is a graph showing the relationship between the pressures applied to the disc brakes and drum brakes in the braking systems of Figures 1 and 2; and Figure 4 is a diagram similar to Figures 1 and 2 showing part of the third embodiment.

Referring to Figure 1, the braking system includes a tandem hydraulic master cylinder 11, a disc brake 12, a drum brake 13 and a failure indicator 14.

The master cylinder 11 comprises a body 15 having a stepped bore, a stepped primary piston 16 and a stepped secondary piston 17.

The primary piston 16 has two spaced lands 18 and 19 by which it is slidable in a larger diameter portion 21 of the stepped bore, and a smaller diameter tubular portion 22 which is slidable in a cylindrical recess 23 in the secondary piston 17 so that the two pistons 16 and 17 telescope together.

The cylindrical recess 23 is in a larger diameter portion 20 of the secondary piston 17 which is slidable in the larger diameter bore portion 21. A smaller diameter portion 30 of the secondary piston 17 is slidable in a smaller diameter portion 24 of the stepped bore, and is connected to the larger diameter portion 20 by a reduced diameter portion having a slot 40 which receives a roll pin 25 which is a push fit in the master cylinder body 15.

A primary chamber 26 is formed between an annular cup seal 27, which abuts the annular end face of the larger diameter portion 20 of the secondary piston 17, and another annular cup seal 28 which seats in a groove in the primary piston 16 between the land 18 and a small flange 29. A first outlet port 31 in the body 15 opens into the primary chamber 26, and is connected to the disc brake 12. With the primary piston 16 in the position shown in Figure 1, a recuperation port 32 being connected to a first inlet port 33 which is for connection to a tank of hydraulic fluid (not shown).

The primary piston 16 has the usual partspherical recess 34 to take thrust from a pushrod (not shown) connected to a driver's brake pedal either directly or through a conventional direct-acting servo.

The recess 23 in the secondary piston 17 forms, with a tubular portion 22 of the primary piston 16, a control chamber 35. The seal 27 acts as a first non-return valve to allow hydraulic fluid from the control chamber 35 to the primary chamber 26, and a metering valve 36 in the primary piston 16 allows, under certain conditions, hydraulic fluid from the control chamber 35 to the first inlet port 33 through a radial passage 37 in the primary piston, and a port 38 parallel to the recuperation port 32.

The metering valve 36 includes a metering valve member in the form of a plunger 39 slidable in a blind bore in the primary piston 16. The plunger 39 has two working diameters, the larger diameter being defined by the outer diameter of a seal 41 retained in a groove in the plunger, and the smaller diameter being defined by the inner diameter of another seal 42 retained by a bearing ring 43, a washer 44 and a circlip 45. An axial bore 46 in the plunger 39 is normally closed by a closure member in the form of a ball 47, biassed by a light compression spring 48, but may be opened by restraining movement of the ball 47 when the plunger is moved away from the washer 43, against the force of a compression spring 49, by means of a pin 51 which is a sliding fit in the plunger. An angled passage 52 in the plunger 39 allows hydraulic fluid through to the radial passage 37.

The secondary piston 17 includes a known type of recuperation valve 53 which is opened by the roll pin 25 when the secondary piston 17 is returned to the position shown in Figure 1 by a return spring 54. The roll pin 25 also provides an abutment for the secondary piston 17 by contacting one end of the slot 40 and, being generally tubular with an axial slit, also provides a passage for hydraulic fluid between a second inlet port 55, connected to the tank of hydraulic fluid, and a secondary chamber 56 formed between the secondary piston and the closed end of the master cylinder body 15. The secondary chamber 56 is connected to the drum brake 13 through a second outlet port 59.Another return spring 57 is provided for the primary piston 16, this acting between a washer 58 and the flange 29, the washer 58 protecting the seal 27 and keeping it in abutment with the annular end face of the large diameter portion 20 of the secondary piston 17. This return spring 57 urges the primary piston 16 against a circlip 61 as shown in Figure 1.

With the components in the positions shown in Figure 1, the pressure in the primary chamber 26, the control chamber 35 and the secondary chamber 56 is in each case that of the tank, and the ball 47 is seated on the end of the bore 46 in the metering valve plunger 39. Action by the driver, which generates a thrust on the part-spherical recess 34, moves the primary piston 16 in the brake applying direction towards the secondary piston 17 so that the seal 28 wipes over the recuperation port 32 to generate a pressure in the primary chamber 26.This primary chamber pressure acts on the seal 27, and thus on a piston area of the secondary piston 17 provided by the annular end face of the larger diameter portion 20, to move the secondary piston in the brake-applying direction towards the closed end of the housing 15 to close the recuperation valve 53 and generate a pressure in the secondary chamber 56.

The rcduction in volume of the control chamber 35, caused by movement of the primary piston 16 towards the secondary piston 17, causes a pressure to build up in this chamber, this pressure being equal to the primary chamber pressure since hydraulic fluid can pass from the control chamber 35 to the primary chamber 26 past the seal 27. Since this fluid is displaced to the disc brake 12 then the total rate of hydraulic fluid displacement through the first outlet port 31 of the disc brake 12 for a given rate of movement of the primary piston 16 relative to the secondary piston 17 is relatively high.

As the primary chamber pressure and the control chamber pressure increase they both act on the metering valve plunger 39 in the direction opposing the biassing load of the spring 49, the primary chamber pressure acting through a radial drilling 62 onto a piston area of the plunger 39 equivalent to the difference in the areas defined by the outer diameter of the seal 41 and the inner diameter of the seal 42, and the control chamber pressure acting on the area defined by the inner diameter of the seal 42. When these pressures have increased to a value which creates a force on the plunger 39 which is sufficient to overcome the load of the spring 49, the plunger and the ball 47 both move towards the primary piston recess 34 until the ball 47 firmly abuts the pin 51.As the ball 47 is restrained against further movement towards the recess 34, the piston area of the plunger 39 which is acted on by the control chamber pressure is now equivalent to the area defined by the inner diameter of the seal 42 less the area defined by the seating diameter of the ball 47. Thus, a small but positive further increase in the primary and control chamber pressures is needed before the primary chamber pressure reaches a first, low, value and the plunger 39 moves further against spring 49 and allows a quantity of hydraulic fluid from the control chamber 35 through to the first inlet port 38.

The immediate effect of this is to lower the pressure in the control chamber 35, so reducing the total fluid pressure load on the plunger 39 acting against spring 49 and allowing the plunger to move back and re-seat the ball 47.

Continuing movement of the primary piston 16 towards the secondary piston 17 causes the plunger 39 to shuttle to and fro, seating and unseating the ball 47 and maintaining the balance of fluid pressure load on the plunger 39 from the primary chamber 26 and the control chamber 35 with the load from the spring 49. Thus, with progressively increasing primary chamber pressure, the control chamber pressure progressively decreases until, at a second, high value of the primary chamber pressure, the control chamber pressure drops to tank pressure and the ball 47 remains un seated for further increases in the primary chamber pressure.

The effect of metering the control chamber pressure is to increase progressively the rate at which the primary chamber pressure in creases with increasing thrust on the pushrod recess 34, thus masking the change in hydraulic displacement ratio which takes place when the metering valve 36 becomes operative.

Once the metering valve 36 operates to reduce the control chamber pressure, hydraulic fluid cannot pass the seal 27 and the displace ment of hydraulic fluid through the first out let port 31 to the disc brake 12 with move ment of the primary piston 16 relative to the secondary piston 17 reduces to a low rate determined by a piston area equivalent to the difference in the areas defined by the outer diameter of the seal 28 and the inner dia meter of the seal 27. To allow the control chamber 35 to fill with hydraulic fluid when the primary piston 16 is returning the ball 47 can unseat from the plunger 39 against the spring 48 to act as a second non-return valve allowing communication from the first inlet port 33 to the control chamber 35.

An area of the end face of the recess 23 in the secondary piston 17 equivalent to the area defined by the outside diameter of the tubular portion 22, is acted on by the control chamber pressure to augment the brake-apply ing force on the secondary piston 17 created by the primary chamber pressure. In the example shown in Figure 1, the secondary piston 17 is dimensioned so that, once the primary chamber pressure has reached the second, high value, it is substantially equal to the secondary chamber pressure. This is achieved by making the piston area of the secondary piston small diameter portion 30 equal to the annular piston area defined be tween the outer and inner diameters of the seal 27.

The effect of the control chamber pressure on the secondary piston 17 is to keep the secondary chamber pressure substantially proportional to the pushrod thrust on the recess 34 in the primary piston 16, so that the vari able ratio effect of the master cylinder is only applicable to that part of the braking system which includes the disc brake 12 and not to the part which includes the drum brake 13.

In a normal four wheel car braking system, there would be two drum brakes 13 to serve the rear wheels and two disc brakes 12 to serve the front wheels. Figure 3 shows a graph of front brake pressure plotted against rear brake pressure which is itself proportional to the mechanical input of the pushrod on the primary piston recess 34. Up to point A, the front brakes receive hydraulic fluid at a pressure proportional to the pedal effort divided by the area of the bore 21, and the rear brakes receive hydraulic fluid at a pressure proportional to the pedal effort divided by the area of the bore portion 24. Consequently, during the initial stage, the front disc brakes receive less pressure than the rear drum brakes.This initial reduction in pressure to the disc front brakes compared to the pressure applied to the drum brakes, allows the shoes of the drum brakes to expand against the force of the usual pull-off springs, so that during light check braking the pads of the disc brakes are not worn unduly. The change in slope at point A in Figure 3 corresponds to the first, low value of the primary chamber pressure when the metering valve 36 starts to open and the change in slope at B corresponds to the second, higher value of primary chamber pressure when the metering valve 36 stops metering and remains open. Thus, between points A and B, the metering valve 36 modifies the pressure reaching the front disc brakes until, at point B, all four brakes receive the same pressurisation.This is because the front brakes receive hydraulic fluid at a pressure proportional to the pedal effort divided by the annular area of the primary piston 16, the rear brakes receive hydraulic fluid at a pressure proportional to the pedal effort divided by the area of the piston 30, and these two areas are arranged to be the same. The effect is, therefore, that the rear brakes are always pressurised in proportion to the pedal effort, whereas the front brakes are buffered until point B so as to be under-pressurised.

The failure indicator 14 avoids problems with the differences between the primary chamber pressure and the secondary chamber pressure, which might cause a normal failure indicator to operate when not required. This indicator 14 includes a stepped plunger 63.

A first piston area is provided by one end face 64 of the plunger 63, and is connected to the master cylinder primary chamber 26. A second piston area is provided by the other end face 65 of the plunger 63, and is connected to the master cylinder secondary chamber 56, and a third piston area is provided by an annular shoulder 66. The piston areas provided by the end faces 64 and 65 and the shoulder 66 are in the same proportional relationship to each other as the piston areas of the secondary piston 17 which are subject to the same pressures, that is to say the primary chamber pressure, the secondary chamber pressure and the control chamber pressure. Hence, the hydraulic forces acting on the indicator plunger 63 are in the same proportional relationship to each other as the hydraulic forces acting on the secondary piston 17.Furthermore, the piston areas on the indicator plunger 63 are arranged so that the hydraulic forces on the indicator plunger 63 have the same directional relationship as the hydraulic forces on the secondary piston 17, so that the indicator plunger 63 is force balanced at all times when the master cylinder is working normally.

A leak or other failure in the braking system, which results in either the primary or the secondary chamber pressure being lower than normal, causes the indicator plunger 63 to move against one or other of the two preloaded centring spring 67 and 68 and so allow a spring loaded plunger 69 of a switch 71 to enter one or the other of two circumferential grooves 72 and 73 in the plunger 63. The switch 71 forms part of an electrical circuit which includes a warning device, such as a lamp or buzzer, which gives a visual or audible warning of the brake system failure to the driver.

The brake system shown in Figure 2 uses a master cylinder 111 which provides a variable ratio effect in a different manner to the master cylinder 111 described above, although the braking system has the same characteristics as that described with reference to Figure 1 in terms of the relationship between the pressures applied to the brakes. The braking system in Figure 2 includes a disc brake 112, a drum brake 113 and a failure indicator 114.

The master cylinder 111 includes a body 115 having a stepped bore, a stepped primary piston 116, and a stepped secondary piston 117.

The primary piston 116 has a large diameter portion 118 slidable in a large diameter portion 121 of the master cylinder body 115, and a small diameter portion 119 slidable within two collars 122 and 123. An annular cup seal 124 provides a seal for the primary piston 116 in the large diameter bore portion 121.

The collars 122 and 123 provide backing for two similar seals 125 and 126 respectively which seal the primary piston small diameter portion 118.

The secondary piston 117 has a large diameter portion 127 slidable in the large diameter bore portion 121 and sealed by an annular cup seal 131, and a small diameter portion 128 slidable in a small diameter bore portion 129 in the housing 115 and sealed by a smaller annular cup seal 132.

A primary chamber 133 is formed between the end faces of the respective large diameter portions 118 and 127 of the primary and secondary pistons 116 and 117. The disc brake 112 is connected to the primary chamber 133 through a first outlet port 134. A secondary chamber 135 is formed between the end face of the secondary piston small diameter portion 128 and the closed end of the small diameter bore portion 129, the drum brake 113 being connected to the secondary chamber 135 through a second outlet port 136.

A control chamber 137 is formed between the large diameter portion 118 of the primary piston 116 and the seal 125. A metering valve 138 is provided in the primary piston 116 for controlling fluid flow from the primary chamber 133 to the control chamber 137, although in Figure 2 the primary piston 116 is shown abutting a stop pin 139 under the effort of a return spring 141 and in this position the primary chamber 133 can communicate with the control chamber 137 through a recuperation port 142, a cavity in the housing 115 sealed by a plug 143 and a passage (not shown) leading from the cavity to the control chamber 137. This passage is parallel to the drilling provided for the stop pin 139.

A first inlet port 144 for connection to a tank of hydraulic fluid (not shown) is connected through a passage 145 in the master cylinder body 115 with a chamber 146 formed between the seals 125 and 126. When the primary piston 116 is in the position shown in Figure 2, the control chamber 137 is open to the chamber 146 through radial ports 147 and 148 in the primary piston, the ports 147 and 148 both opening into a large diameter portion 149 of a stepped bore in the primary piston which stepped bore is provided for the metering valve 138.

The metering valve 138 includes a stepped plunger 151, which is slidable in the large diameter bore portion 149 in the primary piston 116 and in a small diameter bore portion 152. The chamber formed between the smaller end of the stepped plunger 151 and the closed end of the small diameter bore portion 152 is vented into a part-spherical recess 153 in the end of the primary piston smaller diameter portion 119, this recess 153 being provided to receive a thrust from a pushrod (not shown) connected to a driver's brake pedal, either directly or through a direct-acting servo.

The metering valve plunger 151 is biassed away from the recess 153 by a compression spring 154, and into abutment with a washer 155 retained in the stepped bore of the primary piston 116 by a circlip 156. The circlip 156 also provides an abutment for the return spring 141 and the washer 155 also provides an abutment for a light compression spring 157 which biasses a ball 158 into seating engagement with a step in a bore 159 in the plunger 151.

The ball 158 is unseated by a pin 161 when the plunger 151 is moved towards the recess 153 against the load of the spring 154. The pin 161 is a sliding fit in the plunger 151 and abuts a transverse pin 162 which in turn abuts the step between the bore portions 152 and 149. The plunger 151 has a cross-drilling to accommodate the transverse pin 162.

The control chamber 137 is connected, through a port 163 in the plug 143 and a port 164 in the master cylinder body 115, with a chamber 165 formed between the large diameter portion 127 of the secondary piston 117 and a seal 166 which seals the master cylinder large diameter bore portion 121 from the secondary piston small diameter portion 128. This chamber 165 also houses a return spring 167 which biasses the secondary piston 117, into the position shown in Figure 2, against a stop (not shown). In this position a recuperation port 168 is open to the secondary chamber 135, and to a second inlet port 169 for connection to the tank of hydraulic fluid.

When the primary piston 116 is moved in the brake-applying direction towards the secondary piston 117 by a thrust on the recess 153, the initial movement of the seal 124 over the recuperation port 142 cuts off communication from the primary chamber 133 to the control chamber 137 (and hence to the first inlet port 144) and allows the primary piston to pressurise the primary chamber 133, the metering valve 138 being closed at this stage.The primary chamber pressure acts on the piston area of the secondary piston 117 formed by the end face of the large diameter portion 127, to move the secondary piston 117 in the brake-applying direction towards the closed end of the master cylinder housing 115, initial movement of the secondary piston 117 causing the seal 132 to wipe over the recuperation port 168 to cut off communication from the secondary chamber 135 to the second inlet port 169 and so pressurising the secondary chamber 135.

Movement of the primary piston 116 in the brake-applying direction increases the volume of the control chamber 137. While the metering valve 138 is closed, hydraulic fluid enters the control chamber 137 from the chamber 146 past the seal 125, which acts as a non-return valve permitting flow in that direction only. Fluid is displaced from the primary chamber 133 through port 134 to the disc brake 112 at a relatively high rate with respect to the movement of the primary piston 116 towards the secondary piston 117, the effective piston area being that defined by the cross-sectional area of the large diameter bore portion 121 in the housing 115.

The primary chamber pressure acts on the metering valve plunger 151 over an area defined by the cross-sectional area of the larger diameter bore portion 149 of the primary piston 116, and, when sufficient, moves the plunger 151 towards the recess 153 until the ball 158 rests on the pin 161. Primary chamber pressure then rises a small but significant amount since the effective piston area of plunger 151 is now the cross-sectional area of bore portion 149 less the area which is sealed by the ball 158, so that at a first, low value of the primary chamber pressure, the plunger 151 moves further against the spring 154, the pin 161 restraining movement of the ball 158 with the plunger 151 to lift the ball 158 from its seat and allow a quantity of fluid from the primary chamber 133 into the control chamber 137 through the bore 159, radial holes 171 and the ports 148.

The effect of the metering valve 138 allowing a quantity of fluid from the primary chamber 133 into the control chamber 137 is for the control chamber pressure to rise and prevent further flow from the first inlet port 144 past the seal 125. The control chamber pressure acts on the metering valve plunger 151 on a piston are equivalent to the crosssectional area of the large bore portion 149 less the cross-sectional area of the small bore portion 152, to bias the plunger 151 in the same direction as the biassing load of the spring 154 so that the plunger 151 returns to seat the ball 158.

As the primary chamber pressure continues to increase, the metering valve plunger 151 shuttles to and fro, seating and unseating the ball 158 to maintain the balance of hydraulic forces opposing the spring 154, the control chamber pressure rising progressively, and at an increasing rate, until it reaches the same value as primary chamber pressure. At this second, high value of the primary chamber pressure, the load of the spring 154 is balanced by the hydraulic pressure acting on the cross-sectional area of the plunger 151 sliding in the small diameter bore portion 152, and with further increases in the primary chamber pressure the ball 158 remains unseated.

From the point when the metering valve 138 becomes operative to increase control chamber pressure, the ratio of the rate of fluid displacement from the primary chamber 133 through the port 134 to the movement of the primary piston 116 towards the secondary piston 117, changes to a low value determined by the working of the primary piston small diameter portion 119. During return movement of the primary piston 116, the ball 158 unseats to act as non-return valve allowing fluid flow from the control chamber 137 to the primary chamber 133, although the seal 124 can also serve this purpose.

The controlled variation in the control chamber pressure as the primary chamber pressure increases, masks the change in hydraulic ratio when the metering valve 138 becomes operative, the rate at which primary chamber pressure increases with increasing thrust on the pushrod being progressively increased as control chamber pressure rises.

The control chamber pressure in the chamber 165 acts on a piston area of the secondary piston 117 equivalent to the difference in the cross-sectional areas of the master cylinder large bore portion 121 and the small bore portion 129 so as to counteract the brakeapplying force on the secondary piston 117 generated by the primary chamber pressure.

This enables the pressure of the fluid applied to the drum brake 113 to be kept proportional to the driver controlled force of the pushrod on the recess 153 in the primary piston 116 (the pedal effort). In the example of Figure 2 the diameters of the smaller piston portions 119 and 128 are the same, as are the diameters of the larger piston portions 118 and 127, so that the secondary chamber pressure is the same as the primary chamber pressure when the primary chamber pressure has risen above the second, high value. Hence the characteristics shown in Figure 3 also apply to this embodiment of the invention.

The failure indicator 114 has a stepped plunger 172 having working diameters which give piston areas which are proportionally the same as those of the secondary piston 117.

The secondary chamber pressure acts through an inlet port 173 to bias the plunger 172 in one direction, and the primary chamber pressure acts through an inlet port 174 to bias the plunger in the other direction. The control chamber pressure acts through an inlet port 175 partially to counteract the primary chamber pressure in the same way as the control chamber pressure acts in the chamber 165, so that the forces on the plunger 172 are normally balanced. If a failure in the braking system occurs, so that the primary or secondary chamber pressure is reduced or non-existent, then the plunger 172 moves against a centring spring 176 to allow a plunger 177 of a switch (not shown but similar to switch 171) to enter one of two grooves 178 and 179, and send a warning to the driver in a manner as described for Figure 1.

It is, of course, possible for the primary piston to control the rear drum brakes, and the secondary piston to control the front, disc brakes of a vehicle. In this case, the secondary piston would incorporate the metering valve.

Figure 4 shows an embodiment of this type having a secondary piston A which is substantially identical to the primary piston 116 of the embodiment of Figure 2. In view of the similarities between the two embodiments that of Figure 4 will not be described in detail.

The primary piston B of the Figure 4 embodiment is conventional and pressurises hydraulic fluid in a primary chamber C, this pressure being proportional to pedal effort.

The secondary piston A is stepped and has a smaller portion Al which works in the same bore as the primary piston. Consequently, the secondary piston A receives the same mechanical effort as the primary piston B. However, the pressure in the secondary chamber D initially rises at a slower rate than that in the primary chamber C since the effective diameter of the secondary piston at this stage is the diameter of the larger portion A2 of the secondary piston.When the pressure in the secondary chamber D increases to a first, low value, the metcring value E starts to open until, at a second, high value of secondary chamber pressure, the meting valve is com pletely open and the effective diameter of the secondary piston is the inside diameter of the seal H (equivalent to seal 126 of the Figure 2 embodiment), that is to say the diameter of the smaller portion Al. During this stage, the metering valve E smooths the transition from the large effective area of the secondary piston A to the smaller effective area.After the metering valve E stays open at the second, high value of secondary chamber pressure, both the primary chamber C and the secondary chamber D are pressurised to the same extent as the effective area of the secondary piston A equals the area of the primary piston B. Thus, the embodiment of Figure 4 has all the advantages of the two earlier embodiments.

Figure 4 also shows a failure indicator F associated with the secondary piston A. This failure indicator is similar to the indicator 14 and 114 of the earlier embodiment and fulfills the same function.

WHAT WE CLAIM IS:- 1. A vehicle braking system having drum brakes, disc brakes, and a vehicle brake master cylinder comprising first and second pistons acting to pressurise first and second chambers respectively, one chamber controlling the disc brakes and the other chamber controlling the drum brakes, the master cylinder being such that the force transmitted to each piston is directly proportional to the pedal effort applied by the driver throughout the operational range, and wherein the first piston has two effective areas for pressurising the first chamber, the larger area being effective during initial operation of the master cylinder and the smaller area being effective during subsequent operation.

2. A braking system as claimed in Claim 1, wherein the two pistons are mounted inline within a common housing.

3. A braking system as claimed in Claim 2, wherein the driver-controlled effort acts on one of the pistons and the master cylinder is so dimensioned that the force transmitted via the hydraulic fluid to the other piston is directly proportional to the pedal effort.

4. A braking system as claimed in Claim 3, wherein said one piston is the first piston.

5. A braking system as claimed in Claim 3, wherein said one piston is the second piston.

6. A braking system as claimed in any one of Claims 1 to 5, wherein the first chamber is provided with an inlet port connectible to a tank of hydraulic fluid and an outlet port connected to the disc brakes.

7. A braking system as claimed in any one of Claims 1 to 6, wherein the first piston is provided with a metering valve for smoothing out the pressure difference in the first chamber when the effective area of the first piston changes from the large value to the smaller value.

8. A braking system as claimed in Claim 7, wherein the metering valve starts to open at a first, low value of the pressure of the first chamber and is fully open at a second, high value of the pressure of the first chamber.

9. A braking system as claimed in any one of Claims 1 to 8, further comprising a control chamber, movement of the first piston in the brake-applying direction varying the volume of the control chamber.

10. A braking system as claimed in Claim 9, further comprising a first non-return valve allowing fluid flow from the control chamber to the first chamber, and a second non-return valve allowing fluid flow from the inlet port to the control chamber.

11. A braking system as claimed in any one of Claims 8 to 10 when appendant to Claim 7, wherein the metering valve has a metering valve member responsive to the pressure in the first chamber and the pressure in the control chamber, the metering valve member controlling the control chamber pressure.

12. A braking system as claimed in Claim 11, wherein the metering valve member comprises a plunger slidable in a bore in the first piston and biassed towards the closed position of the metering valve by resilient means.

13. A braking system as claimed in Claim 12, wherein the plunger is provided with an axial bore which is normally closed by a closure member, but which is opened by restraining movement of the closure member relative to the plunger in the direction opposing the loading of said resilient means to allow hydraulic fluid to enter or leave the control chamber.

14. A braking system as claimed in Claim 12 or Claim 13, wherein the metering valve is so arranged that the pressure in the first chamber and in the control chamber both act on the plunger in opposition to said resilient means, whereby, until the pressure in the first chamber reaches said, first, low value, the pressure in both the first chamber and the control chamber increases at the same rate with hydraulic fluid passing from the control champ ber to the first chamber through the first nonreturn valve, and, when the metering valve becomes operative at said first, low value, hydraulic fluid passes from the control chamber to the inlet port through the metering valve, the pressure in the control chamber decreasing with increasing pressure in the first chamber until control chamber pressure reaches tank pressure at said second, high value of the pressure in the first chamber.

15. A braking system as claimed in Claim 12 or Claim 13, wherein the metering valve is so arranged that the pressure in the first chamber acts on the plunger in opposition to control chamber pressure and said resilient means, whereby, until the pressure in the first

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. low value, the metcring value E starts to open until, at a second, high value of secondary chamber pressure, the meting valve is com pletely open and the effective diameter of the secondary piston is the inside diameter of the seal H (equivalent to seal 126 of the Figure 2 embodiment), that is to say the diameter of the smaller portion Al. During this stage, the metering valve E smooths the transition from the large effective area of the secondary piston A to the smaller effective area.After the metering valve E stays open at the second, high value of secondary chamber pressure, both the primary chamber C and the secondary chamber D are pressurised to the same extent as the effective area of the secondary piston A equals the area of the primary piston B. Thus, the embodiment of Figure 4 has all the advantages of the two earlier embodiments. Figure 4 also shows a failure indicator F associated with the secondary piston A. This failure indicator is similar to the indicator 14 and 114 of the earlier embodiment and fulfills the same function. WHAT WE CLAIM IS:-

1. A vehicle braking system having drum brakes, disc brakes, and a vehicle brake master cylinder comprising first and second pistons acting to pressurise first and second chambers respectively, one chamber controlling the disc brakes and the other chamber controlling the drum brakes, the master cylinder being such that the force transmitted to each piston is directly proportional to the pedal effort applied by the driver throughout the operational range, and wherein the first piston has two effective areas for pressurising the first chamber, the larger area being effective during initial operation of the master cylinder and the smaller area being effective during subsequent operation.

chamber reaches said first, low value, the control chamber pressure remains at tank pressure by the passage of hydraulic fluid from the inlet port through the second nonreturn valve, and, when the metering valve become operative at said first low value, hydraulic fluid passes from the first chamber to the control chamber through the metering valve, the control chamber pressure increasing with increasing pressure in the first chamber until control chamber pressure equals the pressure in the first chamber at said second, high value.

16. A braking system as claimed in any one of Claims 1 to 15, wherein the second chamber is provided with an inlet port connectible to a tank of hydraulic fluid and an outlet port connected to the drum brakes.

17. A braking system as claimed in Claim 16 when appendant to Claim 4, wherein one piston area of the second piston is subject to the pressure in the first chamber to create a brake-applying force on the second piston, and another piston area of the second piston is subject to pressure in the control chamber to modify the brake-applying force on the second piston created by the pressure in the first chamber, the arrangement being such that hydraulic fluid is displaced from the first chamber through the associated outlet port at a high rate with respect to the brake-applying movement of the first piston relative to the second piston until, when the metering valve becomes operative at said first, low value, said displacement of hydraulic fluid is reduced to a low rate, the control chamber pressure acting on the first piston to increase progressively the ratio of pressure in the first chamber to the driver-controlled pedal effort on the first piston as the pressure in the first chamber increases from said first, low value to said second, high value, and being such that the effect of the control chamber pressure acting on said another piston area of the second piston is to maintain the pressure in the second chamber substantially proportional to said pedal effort.

18. A braking system as claimed in Claim 17 when appendant to Claim 14, wherein said another piston area of the second piston is so arranged that the control chamber pressure acts on the second piston to augment the brake-applying force created by the pressure in the first chamber acting on said one piston area.

19. A braking system as claimed in Claim 18, wherein the first and second pistons are telescoped together so that one end wall of the control chamber is formed by the second piston.

20. A braking system as claimed in Claim 17 when appendant to Claim 15, wherein said another piston area of the second piston is so arranged that the control chamber pressure acts on the second piston to counteract the brake-applying force created by the pressure in the first chamber acting on said one piston area.

21. A braking system as claimed in Claim 16 when appendant to Claim 5, wherein the first piston is stepped and has a portion acting in the same bore as that in which the second piston acts, whereby the force transmitted to the first piston via the hydraulic fluid is directly proportioned to the pedal effort.

22. A braking system as claimed in any one of Claims 3 to 5, or in any one of Claims 6 to 21 when appendant to Claim 3, further comprising an indicator, the indicator comprising a stepped plunger having a first piston area for connection to the chamber associated with said one piston, a second piston area for connection to the chamber associated with said other piston, and a third piston area for connection to the control chamber, the arrangement being such that the pressures in said chambers act on the stepped plunger to produce forces on the stepped plunger which are in the same proportional relationship to each other as the corresponding forces on the other piston produced by the same pressures, the forces on the stepped plunger being in the same directions relative to each other as the corresponding forces on the other piston so that the forces are normally balanced, any failure in the braking system resulting in the pressure in the chambers associated with either of the pistons being lower than normal, causing movement of the plunger to operate a warning device.

23. A vehiclebiraking system substantially as hereinbefore described with reference to, and as illustrated by, Figure 1, Figure 2 or Figure 4 of the accompanying drawings.