GB2240597A - Improvements in hydraulic anti-lock braking systems for vehicles - Google Patents

Abstract

In an hydraulic anti-lock braking system a chamber (16) of a modulator assembly (5) is adapted to act as a dump chamber to relieve brake pressure in an anti-lock mode, and an auxiliary power source (24, 30) causes the chamber (16) to act as a re-application chamber to re-apply the brake following anti-lock correction, solenoid-operated outlet dump valve (17) between the brake (1) and the chamber (16) being cycled so that brake re-application takes place at a controlled rate. The auxiliary source may be a vacuum or compressed air diaphragm device (24) controlled by a solenoid valve (30) and acting on a piston (14) or the piston may be directly acted on by auxiliary pressure. The piston may be spring-loaded to expand the chamber (16), this enabling traction slip control. The piston (14) may be associated with a second, floating piston for multi-circuit control. The modulator may control the front wheels while an accumulator circuit supplies the rear brakes. <IMAGE>

Description

IMPROVEMENTS IN HYDRAULIC ANTI-LOCK BRAKING SYSTEMS FOR VEHICLES This invention relates to hydraulic anti-lock braking systems for vehicles of the kind in which a supply of hydraulic fluid under pressure from actuating means comprising a fluid source of brake-applying pressure, suitably a master cylinder, to a brake on a wheel is modulated by a brake-pressure modulator in accordance with the behaviour of the braked wheel.

Some known anti-lock systems of the kind set forth operate on a two solenoid-operated valves" per channel basis. In such known systems, the solenoid-operated valves usually comprise a first inlet valve between the master cylinder and the brake, which is normally open, and a second outlet valve between the brake and a pressure chamber at lower pressure, suitably an expander or a reservoir.

In an anti-lock mode, the first valve closes to isolate the pressure source from the brake and the second valve opens temporarily to relieve the brake pressure at least by an amount sufficient to prevent excessive angular deceleration of the braked wheel. To reapply the brake pressure, the inlet valve is cycled rapidly to re-admit fluid from the pressure source to the brake via a restrictor. This restrictor interferes with the free transmission of fluid from the source to the brake.

If the actuating means is solely a master cylinder, then the fluid released from the brake must be returned to it by a power source before re-admission of fluid to the brake, otherwise there will be an increase in pedal travel. This return of fluid to the master cylinder may cause adverse pedal vibrations.

Alternatively, the normal master cylinder pressure source in an anti-lock mode may be switched to the boost chamber of an hydraulic servo which derives its energy from a presurised accumulator. In such a construction, the piston of the master cylinder must be held or moved back towards the driver so that if the source of stored energy fails, the brake can still be applied by the master cylinder.

In both systems, the first inlet valve is used to isolate the power source and to re-apply the brake pressure, whilst the second outlet valve is used solely as a means of relieving brake pressure.

When fitting anti-lock braking systems to commercial vehicles where modern hydraulic brakes are employed a problem is encountered with the supply of fluid power to re-apply the brakes due to the large fluid volumes involved. Conventional HGV hydraulic anti-lock braking systems require large and expensive pumps and rotors for at least each axle if not each wheel. Not only is the expense of the power source prohibitive but the actual electric power that needs to be supplied during anti-lock can cause serious problems in the battery/charging circuit and other vehicle safety critical systems. It is an object of the present invention to overcome these difficulties, by avoiding the use of pumps and motors.

According to our invention, in an hydraulic anti-lock system of the kind set forth the modulator comprises a first normally open solenoid-operated valve disposed between the source and the brake, a chamber for fluid, and a second normally closed solenoid-operated valve, the first solenoid-operated valve and the second solenoid-operated valve being movable into closed and open positions respectively in response to an anti-lock signal, whereby the fluid source is isolated from the brake, and fluid in the brake is dumped to the chamber, and upon termination of the anti-lock signal an auxiliary power source is operated to adapt the chamber to act as a brake re-application chamber to re-apply the brake at a rate controlled by cycling the second valve.

In a first embodiment the second normally closed solenoid-operated valve is disposed between the brake and the chamber.

The expander chamber comprises a dump/re-application chamber being switched from a low pressure chamber which receives fluid dumped from the brake, to a chamber acting as a source of high pressure fluid for re-applying the brake, and the second valve is used not only to relieve brake pressure to the chamber, but is also adapted to control the rate at which the brake is re-applied automatically at the termination of the anti-lock signal.

In a second embodiment the chamber is disposed between the first solenoid-operated valve and the second solenoid-operated valve. The expander chamber again comprises a dump/re-application chamber being switched from a low pressure chamber which received fluid dumped from a brake, to a chamber acting as a source of high pressure fluid for re-applying the brake and in this embodiment the second valve is used to relieve the brake pressure from the chamber as well as being adapted to control the rate at which the brake is re-applied automatically at the termination of the anti-lock signal.

In either embodiment switching the chamber between its two operational modes is achieved by the displacement in a bore of a servo piston assembly, and the servo piston assembly may be fitted with a travel transducer to determine its position and the availability of power to re-apply the brakes.

During the anti-lock cycle the first valve remains closed, which provides an excellent pedal feel at the driver's foot.

The first valve, which normally isolates the fluid source from the brake, may also be cycled or pulsed during automatic brake re-application to admit further fluid to the brakes if required.

The auxiliary power source for the chamber may comprise, or be constituted by, any convenient means.

For example the auxiliary power source may be constituted by a pneumatic pressure generating means powered by vacuum generated by the prime mover of the vehicle.

In another system the auxiliary power source may comprise a pressurised hydraulic accumulator, not necessarily associated with the braking system.

A motor, engine, or wheel driven pump, may constitute the auxiliary power source.

In another construction the auxiliary power source may comprise a pressurised air supply, for example a suspension levelling system of the vehicle.

The brake can be applied without restriction at the first solenoid-operated valve (otherwise required by conventional systems). The restriction (if required) is - confined to the second solenoid-operated valve which is normally closed and is not involved with the normal application of brake, except driving anti-lock control.

The modulator assembly may be adapted to control independently of each other the behaviour of brakes on different wheels of a vehicle. In such a construction the modulator assembly comprises a pair of first and second solenoid-operated valves for each brake, with the dump/re-application chamber and the auxiliary power source being common to both channels.

For example, the brakes may be disposed on diagonally opposite front and rear wheels of the vehicle in a system of the 'X' split type, or each channel can control the brakes on both wheels at one end of the vehicle (front/rear split), or on one side of the vehicle (longitudinal split). In these two latter layouts the dump/re-application chamber may be equipped with tandem pistons which constitute the piston assembly to ensure separation of the fluid in the two hydraulic circuits. In the case of a longitudinal split, an apportioning valve may be disposed in the line leading from the modulator assembly to the rear wheel brake of each circuit.

The fluid relieved from one brake during anti-lock control can be transferred to the other to achieve optimum braking on both axles, i.e. the pressure in the over-braked axle is reduced and the pressure in the underbraked axle is increased.

In an alternative arrangement within the scope of the invention an anti-lock braking system may be chosen so that a single power pack of motor, pump and accumulator can energise the anti-lock braking system for each front and both rear brakes even though the brakes are on separate hydraulic circuits. This can normally only be achieved if part of the system has spring loaded deboost type actuators.

The modulators may be further adapted from use as only 'brake slip' or anti-lock control to perform 'drive slip' or traction control.

Some embodiments of our invention are illustrated in the accompanying drawings in which: Figure 1 is a layout of a two-channel anti-lock hydraulic braking system of the 'X' split type for a vehicle; Figure 1A is a layout similar to Figure 1 but adapted for an axle set of brakes; Figure 2 is a layout similar to Figure 1 but adapted for a longitudinal split; Figure 3 is another layout similar to Figure 2 but adapted for a front/rear split; Figure 4 is a system similar to Figure 3 but showing a modification; Figure 5 is a layout similar to Figure 1 but incorporating a different auxiliary power source; Figure 6 is a layout of a triple circuit braking system for a vehicle; Figure 7 is a system similar to Figure 6 but showing a modification; Figure 8 is a system similar to Figure 7 but showing a modification; and Figure 9 is a system similar to Figure 1 but showing a modification.

The braking system illustrated in the layout of Figure 1 is of the 'X' split type in which the behaviour of a brake 1 on a front wheel 2 of a vehicle and the behaviour of a brake 3 on the diagonally opposite rear wheel 4 of the vehicle are adapted to be controlled by a common modulator assembly 5 which is adapted to modulate the supply of fluid to the brakes from a master cylinder 6.

The speed of rotation of each wheel 2, 4 is sensed by a respective speed sensor 7, 8, and signals from the sensors 7, 8 are fed to an electronic control module 9 which differentiate the signals and, in the event of critical signal having been received, emits a electrical current to control operation of the modulator assembly 5 in a manner to be described.

As illustrated, the modulator assembly comprises a housing 11 provided with a pair of first solenoid-operates normally-open inlet valves 12, 13 for controlling, respectively, communication between the master cylinder 6 and the brakes 1 and 3.

A servo-piston 14 works in a bore 15 in the housing 11 and a chamber 16 for fluid, of which the effective volume is variable in accordance with the displacement of the piston 14, is connected to the brakes 1 and 3 through a pair of second solenoid-operated, normally closed, valves 17 and 18 respectively. The servo-piston 14 has a rearwardly extending extension 19 of reduced diameter which is slidably received in a blind bore 20 in an end wall 21 of the housing 11 in which an annular linear travel transducer 22 is accommodated. The transducer 22 is adapted to send signals to the control module 9. A flange 23 extends radially from the piston 14, substantially at the change in diameter, and the flange 23 is backed by a flexible diaphragm 24 which forms a seal between the piston the radial wall of a chamber 25 defined by a space in the housing 11 which is closed by the end wall 21.The diaphragm 24 divides the chamber 25 into a first front variable pressure compartment 26, and a second rear constant pressure compartment 27.

A double-seated, solenoid-operated, switching control valve 30 controls communication between the compartments 26 and 27, and between the compartment 26 and a source of vacuum, suitably a connection to the manifold of the vehicle. Operation of the valve 30 is controlled by an energising current from the control module 9.

In the normal inoperative position illustrated in the drawings, with all the solenoids de-energised the inlet valves 12 and 13 are open so that the master cylinder 6 is in free communication with the brakes 1 and 3, the second solenoid-operated valves 17, 18 are closed so that the brakes 1, 3 are isolated from the chamber 16, and the solenoid-operated control valve 30 is in a position to isolate the compartment 27 from the compartment 26. The diaphragm 24 is therefore subjected to a pressure differential to hold the piston 14 in an advanced position against a stop in the housing 11 and in which the effective volume of the chamber 16 is at a minimum.

When the brake pedal is operated fluid from the master cylinder 6 flows freely to the front and rear brakes 1, 3 through the open valves 12, 13.

If the speed signal from the front wheel 2 indicates a tendency for the wheel to lock, both inlet valves 12, 13 are closed to isolate the master cylinder 6 from both brakes 1 and 3. At the same time the second outlet valve 17 for the front wheel brake 1 is opened, and the switching valve 30 is moved into a position to isolate the compartment 26 from vacuum and equalise the pressures acting across the diaphragm 24.

This enables the piston 14 to move relatively towards the transducer 22 with the result that the effective volume of the chamber 16, which acts as a de-boost chamber, is increased to relieve pressure applied to the brake 1 through the opened valve 17. This action permits the front wheel 2 to recover, at which point the second valve 17 is permitted to close by the control module 9. Simultaneously the solenoid of the switching valve 30 is de-energised, and the pressure differential then re-established across the diaphragm 24 causes the piston 14 to pressurise the volume of fluid trapped in the chamber 16 by the second valve 17.

To re-apply the front brake 1, the second valve 17 is cycled or pulsed by the control module 9 to permit fluid from the chamber 16, which acts, in this mode, as a pressure source defined by re-application chamber, to be supplied to the front brake 1 at a desired controlled rate. The position of the servo-piston 14 is indicated to the control module 9 by the linear transducer 22 so that: 1. Cycling of the outlet valve 17 can be suspended during re-application if the piston 14 does not move or reverses motion, i.e. the pressure level within the re-application chamber 16 is equal to or less than the brake pressure.

2. The outlet valve 18 of the wheel not receiving correction can be cycled towards the end of the travel of the piston 14 to increase pressure to the underbraked wheel 4 and to reduce the pressure applied to the wheel 2 which had to be corrected, i.e. fluid from the overbraked wheel 2 is transferred to the underbraked wheel 4. This transfer of fluid from one wheel to the other is particularly useful in attaining ideal front/rear pressure levels.

3. When brakes are released, the outlet valve 17 can be opened until the servo piston 14 returns to its stop.

Should the signals from both the speed sensors 7 and 8 of the front and rear wheels indicate an incipient skid, both inlet valves 12, 13 are closed or remain closed, and both outlet valves 17, 18 are opened to the chamber 16. The solenoid of the switching valve 30 is also energised. On each wheel recovery the relevant outlet solenoid-operated valve is closed and, subsequently, the two outlet solenoid valves 17, 18 are cycled to re-apply the brake pressure with priority given to the more powerful front brake.

In an anti-lock mode the reaction at the driver's input is minimal since both inlet valves 12, 13 are closed. If, however, the vehicle passes onto a better road surface and the servo piston 14 is in abutment with the stop in the housing 11, the inlet valves 12, 13 are cycled to admit extra master cylinder fluid into the front and rear brakes 1, 3.

Similarly, if the vehicle passes from a high to a low friction surface, the brakes 1, 3 are relieved and are then re-applied solely from the re-application chamber 16.

The anti-lock braking system illustrated in Figure 1A is for an axle set of brakes as opposed to an 'X' split system. The system is intended to be applied to a commercial vehicle where modern hydraulic brakes are employed.

The power supply used is compressed air, and the actuator could be any suitable actuator, for example a conventional fluid master cylinder 6 or an air over hydraulic actuator, where the supply signal from the driver is an air signal converted into a hydraulic brake pressure via a conventional air bag arrangement.

The compressed air of the power supply is normally applied to the back face of the diaphragm 24 by way of the rear compartment 27, while the front face of the diaphragm 24 is exposed to the atmosphere by way of the front compartment 26. Therefore a biasing force exists to hold the boost/deboost piston 14 in the boost position. During a dump cycle the differential across the diaphragm 24 is eliminated by applying compressed air to both sides. This is done by energising the solenoid of switching valve 30. The biasingforce on the piston 14 is removed and the relieved brake pressure fluid can freely expand into the deboost chamber 16.

The areas on both sides of the diaphragm 24 are made equal by sealing the sensor rod 19 and making its diameter D2 the same as the diameter D1 of the deboost piston 14. This is necessary when applying positive pressure to both sides.

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

The anti-lock system illustrated in the layout of Figure 2 is for a longitudinal split braking system in which front and rear brakes 31, 32 and 33, 34 on wheels 35, 36, 37, 38 on opposite sides of the vehicle are applied from separate spaces of a tandem master cylinder 40. An apportioning valve 41 is located in the line leading to each rear wheel brake 32, 34.

A floating piston 42 works in the bore in front of the servo-piston 14 with which it constitutes a tandem servo-piston assembly. The dump/re-application chamber 16 is defined by the two spaces 43, 44 at opposite ends of the floating piston 42, of which the space 42 is connected to the brakes 31, 32 through the second outlet valve 17, and the space 44 is connected to the brakes 33, 34 through the second outlet valve 18.

The provision of the floating piston 42 maintains separation of the two hydraulic circuits.

In the system illustrated in Figure 3 of the accompanying drawings, each of the two master cylinder channels is fed to both front wheel brakes 51, 53, and to both rear wheel brakes 52, 54, respectively, and each wheel has an individual respective wheel speed sensor 55, 56, 57 and 58 which feed signals to the control module 9.

The system of Figure 3 operates on an anti-lock philosophy of 'sense high' fronts and 'sense low' rears. Specifically anti-lock action is taken after the second front wheel tends to lock and, for the rears, anti-lock action is taken when the first rear wheel tends to lock.

The construction and operation of the system of Figure 3 is otherwise the same as that of Figure 2 and corresponding reference numerals have been applied to corresponding parts.

The system illustrated in Figure 4 of the accompanying drawings is similar to Figure 3 but has been modified to enable the front brakes 51 and 53 to be controlled independently of each other. As illustrated the existing solenoid-operated valves 12 and 17 are provided exclusively for the brake 53, and an additional, similar, pair of solenoid-operated valves 60, 61 are provided for the other front wheel brake 51.

Operation of one pair of valves, say the valves 12, 17, as described above, controls operation of the brake 53 independently of the brake 51.

Similarly operation of the valves 60, 61 on their own, control operation of the brake 51 independently of the brake 53.

The system illustrated in the layout of Figure 5 is similar that of Figure 1 but has been modified for use with vehicle already equipped with a power source, for example a compressed air supply for a levelling system or hydraulic power assistance for steering or braking.

As illustrated in Figure 5 the flange 23 and the diaphragm 24 are omitted, and the servo-piston 14 is held in its advanced position by the direct application to a face 70 at the shoulder at the step in diameter of fluid under pressure from the power source under the control of the solenoid operated switching valve 30.

In this construction the switching solenoid is much smaller in size than in the system of Figure 1.

The braking system illustrated in the layout of Figure 6 shows a brake management system with vacuum servo/master cylinder actuation for the front brakes 1, 80 and accumulator 84 powered rears 3. The rear pressure level is controlled by solenoid-operated valves 30, 88 which respond to signals from pressure transducers at the front and rear brakes.

As illustrated a reservoir 81, a motor 82, a pump 83 and a hydraulic accumulator 84 are arranged so as to provide the auxiliary power source.

When the brake pedal is operated, fluid from the master cylinder 6 flows via the two normally open inlet valves 12, 13 to the front brakes 80, 1. The build up in front pressure is fed back to the electronic control module 9 which directs the actuation of the rear solenoid-operated valves 85, 86 to apply the rear brakes 3. The level of rear pressure relative to the fronts is determined by an ideal characteristic stored in the electronic control module 9 based upon initial signals from the front and rear lead cells 92.

The system offers the usual benefits of brake management systems of anti-lock braking front and rear, ideal front/rear brake apportioning, hill hold by application of the rears, and brake intervention for traction is possible by actuating the servo to automatically apply pressure to the fronts.

This system, however, has the additional advantages of a triple circuit braking system with the anti-lock braking controlled from a single power source without mixing the fluids of the three separate systems.

Also there is no pedal reaction during anti-lock braking of front or rear with the resultant advantages of good pedal feel and the use of a standard AS/AS master cylinder.

In the event of a failure of the power source, the front brakes can be applied with servo assistance and without an increase in pedal travel.

The system illustrated in the layout of Figure 7 is similar to that of Figure 6 but the braking of the front wheels is now controlled by a modulator according to the second embodiment of the invention and the tank 81/chamber 44/accumulator 84 connections are now controlled by single seated solenoid-operated valves 100, 101, 102. In the second embodiment of the modulator, the switching solenoid 17, 18, of the first embodiment is used in conjunction with another solenoid-operated valve 101, 102 on the control side of the boost/deboost piston 14, to control both the support chamber 44 deboost and boost. As in Figure 6 the rear brakes 3 are controlled independently by two single seated solenoid-operated valves 85, 86 between the accumulator power source 84 and the brakes 3.

The system illustrated in the layout of Figure 8 is similar to that of Figure 7, but modified to enable the modulators to perform 'drive slip' or traction control. As illustrated a spring 110 acts on the deboost/boost piston 14 in the direction of deboost.

The spring 110 is strong enough to overcome seal friction and fluid resistance.

Normally the accumulator pressure is present in the deboost/support chamber 44. On receipt of a 'wheel spin' signal from one of the given wheels, for example the upper front wheel 87, the drive slip or traction control operates the switch the solenoid-operated valves 17 and 101 into their active positions so closing off the accumulator feed and opening the return line to the reservoir 81. This then allows the 'light' spring 110 in the chamber 43 to expand and draw fluid into the chamber 43 from the master cylinder 6 through the open solenoid-operated valve 12. When the deboost/boost piston 14 has travelled to its fully expanded position the switch 91 engages and emits a signal to the controller 9 that causes the solenoid-operated valve 12 to close, isolating the master cylinder 6 from the brake 80.Brake pressure can then be applied from the accumulator 84 controlled by the solenoid-operated valves 17 and 101 through what is now effectively a master cylinder piston (the deboost/boost piston 14). When the wheel 87 has stopped spinning the solenoid-operated valve 12 is opened and full accumulator pressure is applied to the chamber 44, this then returns the withdrawn fluid back to the master cylinder 6.

Such a spring may be added to any of the specific valve configurations so far considered to move the deboost/boost piston 14 to its expanded position.

Figure 9 shows a layout similar to that of Figure 1 but modified to include such a spring.

If the front upper wheel 87 passes onto a slippery surface (whilst the brakes are applied) and a skid condition is sensed, the inlet valve 12 closes to isolate the master cylinder 6 from the front brake 80.

Simultaneously, the outlet 17 and upper double-seated valve 88 are energised. In consequence, the pressure in the upper front brake 80 is relieved by the movement of the expander piston 14 until the wheel recovers. On wheel recovery, the outlet 17 and double-seated valve 88 are de-energised. As a result, the accumulator 84 acts upon the expander piston 14 urging it to the left, thus pressurising the fluid trapped between the other side of the piston 14 and the closed outlet valve 17. Subsequently, the outlet valve 17 is cycled or pulsed to re-apply the upper front brake pressure until the expander piston 14 has travelled fully back (sensed by a position switch 91). The inlet valve 12 may now open to reconnect the master cylinder 6 with the brake 80.

For rear anti-lock braking, the inlet valve 85 is closed and the outlet valve 86 opens to relieve rear brake pressure to the reservoir. On wheel recovery, the inlet valve 85 is cycled or pulsed to re-apply fluid under pressure for the accumulator 84 to a level predetermined by the electronic control module 9.

Claims (20)

1. An hydraulic anti-lock system of the kind set forth in which a brake pressure modulator comprises a first normally open solenoid-operated valve disposed between a fluid source and a brake, a chamber for fluid, and a second normally closed solenoid-operated valve movable into closed and open positions respectively in response to an anti-lock signal, whereby the fluid source is isolated from the brake, and fluid in the brake is dumped to the chamber, and upon termination of the anti-lock signal an auxiliary power source is operative to adapt the chamber to act as a brake re-application chamber to re-apply the brake at a rate controlled by cycling the second valve.

2. An anti-lock system according to claim 1, in which the second normally closed solenoid-operated valve is disposed between the brake and the chamber.

3. An anti-lock system according to claim 1, in which the chamber is disposed between the first solenoid-operated valve and the second solenoid-operated valve.

4. An anti-lock system according to claim 2 or claim 3, in which the chamber comprises a dump/re-application chamber being switched from a low pressure chamber which receives fluid dumped from the brake, to a chamber acting as a source of high pressure fluid for re-applying the brake, and the second valve is adapted to control the rate at which the brake is re-applied automatically at the termination of the anti-lock signal.

5. An anti-lock system according to any preceding claim, in which the chamber is switched between its two operational modes by the displacement in a bore of a servo piston assembly.

6. An anti-lock system according to any preceding claim, in which the first normally open solenoid-operated valve is also cycled during automatic brake re-application to admit further fluid to the brake when required.

7. An anti-lock system according to any preceding claim, in which the modulator assembly is adapted to control independently of each other the behaviour of brakes on different wheels of a vehicle.

8. An anti-lock system according to claim 7, in which the modulator assembly comprises a pair of first and second solenoid-operated valves for each of the two brakes, with the chamber and the auxiliary power source being common to both channels.

9. An anti-lock system according to claim 8, in which the chamber is equipped with tandem pistons which constitute the piston assembly to ensure separation of the fluid in the two hydraulic circuits.

10. An anti-lock system according to claim 9, in which an apportioning valve is disposed in a line leading from the modulator assembly to a rear wheel brake of each circuit.

11. An anti-lock system according to claim 7, in which a single power pack of motor, pump and accumulator is used to energise the anti-lock systems of the separate hydraulic circuits.

12. An hydraulic anti-lock braking system for a vehicle substantially as described herein with reference to and as illustrated in Figure 1 of the accompanying drawings.

13. An hydraulic anti-lock braking system for a vehicle substantially as described herein with reference to and as illustrated in Figure 1A of the accompanying drawings.

14. An hydraulic anti-lock braking system for a vehicle substantially as described herein with reference to and as illustrated in Figure 2 of the accompanying drawings.

15. An hydraulic anti-lock braking system for a vehicle substantially as described herein with reference to and as illustrated in Figure 3 of the accompanying drawings.

16. An hydraulic anti-lock braking system for a vehicle substantially as described herein with reference to and as illustrated in Figure 5 of the accompanying drawings.

17. An hydraulic anti-lock braking system for a vehicle substantially as described herein with reference to and as illustrated in Figure 6 of the accompanying drawings.

18. An hydraulic anti-lock braking system for a vehicle substantially as described herein with reference to and as illustrated in Figure 7 of the accompanying drawings.

19. An hydraulic anti-lock braking system for a vehicle substantially as described herein with reference to and as illustrated in Figure 8 of the accompanying drawings.

20. An hydraulic anti-lock braking system for a vehicle substantially as described herein with reference to and as illustrated in Figure 9 of the accompanying drawings.