GB2227061A

GB2227061A - Hydraulic anti-lock braking systems for vehicles

Info

Publication number: GB2227061A
Application number: GB8924377A
Authority: GB
Inventors: Glyn Phillip Reginald Farr
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1988-11-04
Filing date: 1989-10-30
Publication date: 1990-07-18
Also published as: JPH02175365A; DE3936697A1; GB8924377D0

Description

4 1 IMPROVEMENTS IN HYDRAULIC ANTI-LOCK BRAKING SYSTEMS FOR VEHICLES This

invention relates to hydraulic anti-lock braking systems for vehicles of the kind inwhich hydraulic operating f luid from a supply, suitably an hydraulic master cylinder, to a brake is modulated by a modulator assembly in accordance with an anti-lock signal from wheel speed sensing means, and a supply of hydraulic fluid under pressure from a source is provided to control brake re-application following emission of an anti-lock signal, valve means being operative in response to an anti-lock signal to cause the supply to be isolated from the brake, and the pressure of fluid applied to the brake to be relieved, and after the wheel has recovered to enable the brake to be re-applied at a controlled rate.

In one known anti-lock braking system of the kind set forth the control valve means comprises separate independently operable solenoid-operated inlet and exhaust valves. In normal operation fluid is supplied to the brake through an orifice defined by the seating of the inlet valve. Due to brake volume and application rate considerations during anti-lock control, the inlet and exhaust valves are desirably relatively small in size which, in turn, dictates the size of the orifice. In some constructions therefore the orifice is relatively small and, as a consequencef may become blocked by the presence of foreign matter in the fluid, resulting in failure of the applying force to the brake.

In another known anti-lock braking system of the kind set forth, and in accordance with EP-A-0 202 845, the control valve means comprises one solenoid-operated 2 valve in combination with a flow-control regulator valve. The f low- control regulator valve permits f ree flow of fluid in a normal brake applying situation, but is movable to introduce a restriction between the master cylinder and the brake in an anti-lock mode. Since the flow- control regulator valve comprises a spool working in a bore in a housing, it is essential for the clearances between the spool and the bore to be kept to a minimum to prevent uncontrolled leakage through a portion of the valve which isolates the master cylinder from the brake when the valve acts as a cut-off valve.

According to our invention, in an anti-lock hydraulic braking system of the kind set forth a dump valve responsive to an anti-lock signal is normally closed to isolate the brake from an expander chamber of which the effective volume is determined by movement of an expander piston in a bore and is normally at a minimum, an isolating valve between the supply and the brake is normally held in an open position by the expander piston to' permit free and unrestricted flow from the supply to the brake, movement of the dump valve into an open position in response to an anti- lock signal causing the expander piston to retract and increase the effective volume of the expander chamber initially to permit the isolating valve to close and is olate the supply from the brake and thereafter to relieve the pressure applied to the brake.

Since the unrestricted free flow for normal brake operation passes through a chamber incorporating the dump valve, bleeding of the main brake line is facilitated.

3 Conveniently the isolating valve is adapted to close after the expander piston has first moved through a predetermined distance against the load in a bias spring.

is The dump valve is arranged in series with an inlet valve which is located in a line between the supply and the dump valve and which is by-passed by the isolating valve when the isolating valve is in its open position.

The inlet valve may comprise a flow control regulator valve or it may comprise a normally-open cut-off valve.

When the inlet valve comprises a flow control regulator valve, the flow valve is responsive to a difference in pressure between the supply and the brake and is adapted to meter fluid to the brake at a controlled rate. The flow valve is therefore inoperative when the isolating valve is open so that erroneous movement of the flow valve caused by trapped pockets of air does not interfere with the main line flow between the supply, suitably a master cylinder, and the brake.

Since the flow valve only controls the pressure rise rate in an anti-lock mode and the separate isolator valve isolates the supply from the brake, the clearances between a spool and a bore in which it works and which constitutes the flow valve can be increased. Specifically the clearance tolerance between the spool and the bore can be relatively large. This is because we can arrange for the relative surface contact areas to be relatively large and because excessive leakage at 4 a metering edge of the spool can be compensated for by further movement of the flow valve.

These large surface contact areas can be provided since the f low valve acts only as a metering valve and not as both a cut-off valve and a metering valve.

When the inlet valve comprises a normally-open cut-of f valve, suitably a solenoid-operated valve, the open isolating valve ensures free unrestricted transmission of fluid to the brake for normal brake-applying purposes, without any restriction due to the presence of the seating of the inlet valve.

The energy required to recover f luid in order to re-apply the brake at the termination of an anti-lock signal is provided, in a known manner, by a pump which withdraws fluid from the expander chamber and pumps it to the master cylinder. The output from the pump is fed to the inlet of the inlet valve so that the differences between the pump flow and the flow through the inlet valve are felt by the master cylinder which constitutes the supply.

Preferably a restrictor is disposed between the master cylinder and the inlet to the inlet valve whereby to reduce the reaction at the master cylinder and provide the driver with a good pedal feel.

In one construction the isolating valve comprises a piston working in a bore in a housing and having a radial flange which constitutes a valve head and is normally spaced from a seating disposed between the master cylinder and the dump valve, the isolating valve being held in its open position with the head spaced from the seating by a piston-rod co-operating with the expander piston.

When the inlet valve comprises a flow control regulator valve, the flow control regulator valve may comprise a spool which works in a bore in the piston and is provided with a fixed orifice to regulate flow through a longitudinal bore in the spool, and a metering edge at one end of the spool co-operates with a port in the wall of the piston to def ine a variable orifice which determines the rate at which the brake can be re-applied.

is Conveniently the restrictor between the master cylinder and the inlet to the flow valve comprises an orifice in the end of the piston remote from the expander piston.

When the inlet valve comprises a cut-off valve, the valve comprises a solenoid-operated valve member for engagement with a seating in a housing, and the restrictor comprises a restricted orifice in the housing disposed between the inlet from the master cylinder and the seating, with the output from the pump being connected between the restrictor and the seating.

The dump valve may comprise a solenoid-operated valve, and the pump may be driven by an electric motor, both the solenoid-operated valve and the pump being operated in sequence by a control module in accordance with signals from the wheel speed sensing means.

Two modulator assemblies may be incorporated in a single unit connected to a common master cylinder and a 6 common pump. This enables independent control of front and rear brakes to be achieved.

A single expander chamber, expander piston, and isolating valve may be provided for the front and rear brakes. In such a construction the pressure applied to one of the pairs of front and rear wheels can increase at a rate determined by flow through the restrictor when the other pair is subject to anti-lock control.

Preferably however two expander chambers are interconnected through a one-way valve which allows flow from the expander chamber associated with the front wheel to the other expander chamber, but prevents flow in the opposite direction. This ensures that the behaviour of the rear wheels can be controlled independently of the fronts, but control of the front wheels reduces the rate at which pressure applied to the brakes on the rear wheels can increase, thereby reducing any tendency for the rear wheels to lock.

Some embodiments of our invention are illustrated in the accompanying drawings in which:- Figure 1 is a layout of a braking system including a single modulator assembly; Figure 2 is incorporating construction; a layout similar to Figure 1 but a modulator assembly of dual Figure 3 is a layout similar to Figure 1 but showing a modification; and 7 Figure 4 is a layout similar to Figure 3 but incorporating a modulator assembly of dual construction.

In the anti-lock braking system illustrated in Figure 1 of the accompanying drawings the supply of fluid under pressure from a pedal-operated master cylinder 1 to a brake 2 on a wheel 3 is modulated by a modulator assembly 4, and a pump 5 driven by an electric motor 6 provides the energy to replenish the fluid to re-apply the brake 2 automatically following an anti-lock mode.

The modulator -assembly 4 comprises a housing 7 provided with a solenoid-operated dump valve 10, an expander chamber 11 defined between an expander piston 12 and a bore 13 in which it works, an isolating by-pass valve 14, and an inlet valve comprises a flow-control regulator valve 15.

The isolating by-pass valve 14 comprises a piston 16 working in a bore 17 in the housing 7 and which is co-axial with and spaced from the bore 13 by a wall 18, and the piston 16 is provided with a radial flange 20 carrying an annular seal 21. The seal 21 constitutes a valve head for engagement with a seating 22 disposed between an inlet passage 23 from the master cylinder 1 and the solenoid-operated valve 10. Normally the head 21 is urged away f rom the seating 22 against the load in spring 24 by means of a spring 30, which holds the piston 12 in an advanced position in which the effective volume of the chamber 11 is at a minimum, the piston 12 acting on the head 21 through an integral projection in the form of a rod 31 which extends through a sealed bore 32 in the wall 18.

8 The flow-control regulator valve 15 comprises a spool 33 which works in a fluid bore 34 in the piston 16, and the bore 34 is in communication with the pump 5 through a passage 35 in the piston 16, and with the master cylinder 1 through a restricted orifice 36 in the piston 16. The spool 33 has a longitudinal bore 37 connecting with the brake 3 through a restricted passage in the form of a fixed orifice 38 in the adjacent end of the spool 33, and a spring 39 normally urges the spool 33 relatively away from the piston 12. In this position a metering edge 40 at that end of the spool 33 is spaced from a radial port 41 in the wall of the piston 16, and the edge 40 co-operates with the port 41 to def ine a variable orifice when the spool 33 is advanced in the bore 34 in response to a pressure drop across the orifice 38.

The pump 5 has an inlet and an outlet of which the inlet is connected to the expander chamber 11 through a one-way inlet valve 42, and the outlet is connected to the passage 35 through a one-way outlet valve 43.

The behaviour of the wheel 3 is sensed by a speed sensor 45, and the sensor 45 sends signals to an electronic control module 46. The module 46 differentiates the signals and, if critical, operates the solenoid-operated valve 10 and the electric motor 6 in a manner to be described.

In a normal brake-applying position the solenoid of the valve 10 and the motor 6 are both de-energised.

In this position the solenoid-operated valve 10 is closed to isolate the brake 2 from the expander chamber 11. The isolating valve 14 is held open by the piston 12 so that there is a free and unrestricted communication between the master cylinder 1 and the 9 brake 2 for brake application, brake- release, and brake bleeding purposes. When the brake 2 is applied, pressure builds up at the outer ends of the orifices 36 and 38 and pressurises the outlet valve 43.

When an anti-lock signal is emitted by the sensor 45 only the solenoid of the valve 10 is energised to open the dump valve 10 which admits brake fluid into the expander chamber 11. This causes the piston 12 to retract against the force in the spring 30 which, in turn, permits the isolating valve 14 to close with the head 21 engaging with the seating 22 to isolate the master cylinder 1 from the brake 2.

pressure applied to the brake 2 falls, valve 15 moves against the spring 39 due pressure drop across the fixed orifice 38, and the metering edge 40 hovers at the edge of the radial port 41 to meter fluid to the brake 2, but at a controlled rate. Simultaneously the motor 6 is started to operate the pump 5, and f luid is forced back from the expander chamber 11 towards the flow valve 15.

As the the f low to the The output f low from the pump 5 is designed to be substantially equal to that metered by the flow valve 15 so that the reaction at the master cylinder 1 is small. This reaction is further reduced by the restricted orifice 36.

When the wheel 3 recovers, the solenoid-operated valve 10 closes to isolate the brake 2 from the expander chamber 11, and the pump 5 is operative to recover fluid from the expander chamber 11. Simultaneously the pressure applied to the brake 2 is increased, but at a rate determined by the flow valve 15, and the flow rate is, in turn, determined by the force in the spring 39 acting upon the spool 33, and the size of the orifice 38.

During brake re-application the pump 5 continues to draw fluid from the expander chamber 11 until the projection 31 abuts the isolating valve 14. Due to the difference in pressure between that of the master cylinder 1 and the brake 2, the isolating valve 14 initially remains closed, but opens when the pressure applied to the brake 2 is substantially equal to the input pressure from the master cylinder 1.

The clearance tolerance between the spool 33 and the bore 34 can be fairly large because of the is relatively long length of contact area between the spool 33 and the bore 34, and because excessive leakage at the metering edge 40 can be compensated for by further movement of the flow valve 15.

In the braking system illustrated in the layout of Figure 2, two modulator assemblies 50,51 are housed in parallel within a common housing 52. Each assembly is similar to the assembly of Figure 1 and comprises a solenoid-operated dump valve 53,531, an expander chamber 54,54' defined between an expander portion 55,551 and a bore 56,561 in which it works, an isolating valve 61,61', and a flow-control regulator valve 57,571.

The master cylinder 1 is connected to both modulator assemblies 50,51 through the connection 23 and a cross-passage 58, and the outlet from the pump 5 is connected to the passage 35 through a similar passage 351 in the isolating valve 61 and a cross-passage 59. A further cross passage 60 11 interconnects the outer ends of the two isolating valves 61,611.

The two expander chambers 54,541 are connected by a one-way valve 63 which permits fluid to flow from the chamber 54 into the chamber 541. but prevents flow in the opposite direction. Chambers 64,64' on the opposite sides of the pistons 55,551 are interconnected by the passage 65.

The modulator assembly 50 is connected to a brake 70 on a front wheel 71, and the modulator assembly 51 is connected to a brake 72 on a rear wheel 73. The arrangement is such that independent is control of the front wheel 71 and the rear wheel 73 is achieved.

Each wheel has a wheel speed sensor 74,74' from which signals are sent to the control modulate 46.

In normal brake operation both valves 53,53' are closed, the pump 5 is inoperative, and the components are otherwise in the position shown in Figure 2 of the drawings.

When the master cylinder 1 is operated, both brakes 70,72 are applied simultaneously with the pressures equalised by compensating flow through the passages 58,59,60.

When both speed sensors 74 and 741 emit anti-lock signals simultaneously, both modulators 50,51operate in a similar manner to that described above with reference to Figure 1. Specifically the modulators 50,51 are operative to relieve the pressures applied to the brakes 70,72, and re-apply them 12 automatically at the terminations of the anti-lock signals.

However, should only the sensor 741 emit an anti-lock signal, the solenoid-operated valve 531 is opened to initiate operation of the modulator assembly 51 to modulate the supply of f luid to the rear brake 73 in a manner similar to that described above with reference to Figure 1. Since the expander chamber 54 is at a pressure either equal to or lower than that in the expander chamber 541, due to the anti-lock mode of the rear wheel brake 72 the one-way valve 63 prevents the higher pressure in the expander chamber 54' from passing into the expander chamber 54.

is Anti-lock control of the rear brake 72 is therefore independent of the front brake 70, and the operation of the front brake 70 is unaffected by anti-lock control of the rear brake 72.

On the other hand, when an anti-lock signal is emitted by the sensor 74 only, the operation of the modulator assembly 50 is initiated by energising the solenoid-operated valve 53 and the modulator assembly 50 operates as described above with reference to Figure 1. The higher pressure dumped into the expander chamber 54 also passes through the oneway valve 63 into the expander chamber 5V. The expander piston 551 then retracts to permit the isolating valve 611 to close. Since the solenoidoperated valve 531 remains closed, a flow path from the master cylinder 1 and the pump 5 to the rear wheel brake 72 is maintained through the flowcontrol valve regulator 571. The pressure applied to the rear brake 72 continues to rise but at an reduced rate of pressure increase.

1 1 13 In the braking system illustrated in the layout of Figure 3, the inlet valve comprises a solenoid-operated cut-off valve 80 which replaces the flow-control regulator valve 15.

The solenoid-operated cut-off valve 80 comprises a solenoid-operated valve member 81 for engagement with a seating 82 at the outer end of a passage 83 in the housing 7. The passage 83 leads from the bore 17 through a restrictor 84. The output from the pump 5 is connected to the passage 83 between the restrictor 84 and the seating 82.

The isolating valve 14 comprises a head 85 which is carried by the free end of the projection 31, and is urged towards the seating 22 by a compression spring 86.

The pump 5 is also provided with a pressure relief valve 87.

in a normal brake-applying position the solenoids of both valves 10 and 80, and the motor 6 itself are de-energised, the inlet valve 80 is open, and the solenoid-operated valve 10 is closed to isolate the brake 2 from the expander chamber 11. The isolating valve 14 is held open by the piston 12 so that there is a free and unrestricted communication between the master cylinder 1 and the brake 2 for brake application, brake release, and brake bleeding purposes. When the brake 2 is applied by the master cylinder 1, pressure builds up at the outer end of the restrictor 84, pressurises the outlet valve 43 from the pump 5, and acts upon closed dump valve 10.

0 14 When an anti-lock signal is emitted by the sensor 45, the solenoid of the inlet valve 80 is energised to close the inlet valve 80. The dump valve 10 is opened to admit brake fluid into the expander chamber 11. This causes the piston 12 to retract against the force in the spring 30. This, in turn, permits the isolating valve 14 to close with the head 85 engaging with the seating 22 which isolates the master cylinder 1 from the brake 2. As the pressure applied to the brake 2 falls, the wheel 3 recovers and the dump valve 10 closes again.

is The receipt of the skid signal at the control module 46 also instigates energisation of the motor 6 to drive the pump 5. This draws fluid from the expander chamber 11, and pumps it back into the space between the restrictor 84 and the inlet valve 80.

When the inlet valve 80 is closed the pump 5 will meet the resistance of the restrictor 84, and the relief valve 87 will open to reduce the output from the pump 5. However, when the inlet valve 80 is cycled by the control module 46 to re-apply the brake 2 by pressure from the pump 5, only the output from the pump 5 minus the flow to the brake 2 is fed to the master cylinder 1. Thus the restrictor 84 between the pump 5 and the master cylinder 1 will give a good pedal feel but it will not interfere with normal brake application.

The construction and operation of the system illustrated in Figure 3 is otherwise the same as that of Figure 1, and corresponding reference numerals have been applied to corresponding parts.

is In the braking system illustrated in the layout of Figure 4, the modulator assembly 4 is modified to control the behaviour of a brake 90 on a rear wheel 91 diagonally opposite the front wheel 3 provided with the brake 3, and the behaviour of the wheel 90 is sensed by a skid sensor 92 which feeds skid signals to the control module 46.

As illustrated, the modulator assembly 4 is provided with an additional rear inlet valve 80' and an additional rear dump valve 10, which are connected together in series. These valves 80' and 10' are both solenoid-operated, are each similar in construction to the inlet valve 80 and the dump valve 10, respectively, and are both operated by energising currents from the control module 46.

The output from the pump 5 is connected to the inlet of the inlet valve 80' through a branch passage 93, and the dump valve 101 is normally closed to close a second outlet passage from the expander chamber 11.

In an inoperative position the four solenoids and the motor 6 all are de-energised so that the inlet valves 80 and 80' are open, the dump valves 10 and 10' are closed, and the pump 5 is inoperative. As before, the isolating by-pass valve 14 is held open by the expander piston 12.

When the master cylinder 1 is applied, fluid flows freely to the front brake 2 through the open by-pass valve 14 and past the closed dump valve 10. In addition fluid flows freely to the rear brake 90 back through the open inlet valve 80, the branch passage 93, the open inlet valve 80', and past the dump valve 10'.

16 If a skid signal is received at the front wheel 3 only, the control module 46 is operative to close the inlet valve 80 and open the dump valve 10. Opening the dump valve 10 relieves f luid from the brake 2 to the expander chamber 11. This causes the piston 12 to retract and close the isolating valve 14 as described above. After closure of the isolating valve 14, the pressure applied to the rear brake 90 can still be increased from the master cylinder 1 but at a rate 10 determined by flow through the restrictor 84.

Upon recovery of the front wheel 3, the control module 46 causes the dump valve 10 to close and it also energises the motor 6. The pump 5 then draws fluid from the expander chamber 11 and forces it into the space between the inlet valve 80 and the restrictor 84. Since the inlet valve 80' is open, this fluid can also flow to the rear brake 90 to increase still further the pressure applied to that brake 90.

To re-apply the front brake 2, the control module 46 is operative to close the inlet valve 801, and cycle the inlet valve 80 at a predetermined rate to give a f low which is similar to the output from the pump 5. The pedal reaction is therefore minimal.

If a skid signal is received at the rear wheel 91 only, the rear inlet valve 801 is closed and the dump valve 101 opens to connect the rear brake 90 to the expander chamber 11. In consequence, the expander piston 12 moves to close- the isolating valve 14. The pressure applied to the front brake 2 can still increase further from the master cylinder 1 but at a rate controlled by flow through the restrictor 84 and the open inlet valve 80.

11 X.

17 On recovery of the rear wheel 91, the rear dump valve 101 is closed and the pump 5 draws fluid from the expander chamber 11. At the same time the inlet valve is pulsed to re-apply the rear brake 90 at a controlled rate.

Should both the front wheel 3 and the rear wheel 91 receive a skid signal, the control module 46 is operative to close the two inlet valves 80, 80', and open the two dump valves 10, 10', thereby to relieve some of the pressures in the two brakes 2 and 90. if these pressures are held constant during wheel recovery. that is to say with all four solenoid- operated valves 80, 10, 811 and 10' closed, the output f rom the pump 5 is reduced by opening the relief valve 87 to provide a reasonable pedal feel.

The construction and operation of the system illustrated in Figure 4 of the drawings is otherwise the same as that of Figure 3, and corresponding reference numerals have been applied to corresponding parts.

In the systems of Figures 3 and 4 described above the restrictor 84 not only acts to dampen oscillations of the pump 5, but also serves as a flow control orifice for the re-apply rate controlled by the solenoid-operated valve 80 or the valves 80, 811.

In the system of Figure 4, the pump 5 can direct its output to the rear brake 90, whilst the front inlet valve 80 is closed, or to the front brake 2, whilst the rear inlet valve 80 is closed. The brakes 90, 2 are therefore used as attenuation chambers in order to smooth out cyclic characteristics of the pump 5.

k_ it

Claims

CLAIMS is 1. An hydraulic anti-lock braking system for vehicles in which a

dump valve responsive to an anti-lock signal is normally closed to isolate the brake from an expander chamber of which the effective volume is determined by movement of an expander piston in a bore and is normally at a minimum, an isolating valve between the supply and the brake is normally held in an open position by the expander piston to permit free and unrestricted flow from the supply to the brake, movement of the dump valve into an open position in response to an anti-lock signal causing the expander -piston to retract and increase the effective volume of the expander chamber initially to permit the isolating valve to close and isolate the supply from the brake and thereafter to relieve the pressure applied to the brake.
2. A system according to claim 1, in which for normal brake operation unrestricted flow passes through a chamber incorporating the dump valve.
3. A system according to claim 1 or claim 2, in which the isolating valve is adapted to close after the expander piston has first moved through a predetermined distance against the load in a bias spring.
4. A system according to any preceding claim, in which the isolating valve comprises a piston working in a bore in a housing and having a radial f lange which constitutes a valve head and is normally spaced from a seating disposed between the supply and the dump valve, the isolating valve being held in its open position with the head spaced from the seating by a piston-rod co-operating with the expander piston.

k) 4, c - 19
5. A system according to claim 4, in which the dump valve is arranged in series with an inlet valve which is located in a line between the supply and the dump valve and which is by-passed by the isolating valve when the isolating valve is in its open position.
6. A system according to claim 5, in which the inlet valve comprises a flow control regulator valve responsive to a difference in pressure between the supply and the brake and adapted to meter f luid to the brake at a controlled rate.
7. A system according to claim 5, in which the inlet valve comprises a normally-open cut-off valve.

is
8. A system according to claim 7, in which the inlet valve is solenoidoperated.
9. A system according to any of claims 5-8, in which a pump is operative at the termination of an anti-lock signal to withdraw fluid from the expander chamber and pump it to the supply, and a restrictor is disposed between the supply and the inlet to the inlet valve.
10. A system according to claim 6, in which the f low control regulator valve comprises a spool which works in a bore in the piston and is provided with a f ixed orif ice to regulate f low through a longitudinal bore in the spool, and a metering edge at one end of the spool co-operates with a port in the wall of the piston to define a variable orifice which determines the rate at which the brake can be re-applied.
11. A system according to claim 10, in which a restrictor between the supply and the inlet to the flow 4, valve comprises an orifice in the end of the piston remote from the expander piston.
12. A system according to claim 9, in which the inlet valve comprises a solenoid-operated valve member for engagement with a seating in a housing, and the restrictor comprises a restricted orifice in the housing disposed between the inlet from the supply and the seating, with the output from the pump being connected between the restrictor and the seating.
13. A system according to claim 9, in which the dump valve comprises a solenoid-operated valve, and the pump is driven by an electric motor, both the solenoid-operated valve and the pump being operated in sequence by a control module in accordance with signals from the wheel speed sensing means.
14. A system according to any preceding claim, characterised in that two modulator assemblies are incorporated in a. single unit connected to a common supply and a common pump to achieve independent control of at least one front wheel brake and one rear wheel brake.
15. A system according to claim 14, in which that a single expander chamber, expander piston, and isolating valve (14) are provided for the front and rear brakes.
16. A system according to claim 14, in which two expander chambers are interconnected through a one-way valve which allows flow from the expander chamber associated with the front wheel to the other expander chamber but prevents flow in the opposite direction.

1 is 21
17. An hydraulic anti-lock - braking system substantially as described herein with reference to and as illustrated in Figure 1 of the accompanying drawings.
18. An hydraulic anti-lock braking system substantially as described herein with reference to and as illustrated in Figure 2 of the accompanying drawings.
19. An hydraulic anti-lock braking system substantially as described herein with reference to and as illustrated in Figure 3 of the accompanying drawings.
20. An hydraulic anti-lock braking system substantially as described herein with reference to and as illustrated in Figure 4 of the accompanying drawings.

Published 1990 at The Patent Office. State House. 66 71 High Holborn. London WC1R4TP-F'urther copies maybe obtainedfrcn The Patent OfficeSales Branch. St Mary Cray. Orpington. Kent BR5 3RE Printed by MiAt3plex tech2uqjes ltd. St Mary Cray. Kent. Con 1 87