GB2128699A - Improvements in hydraulic anti- skid braking systems for vehicles - Google Patents

Abstract

Operating fluid from a supply (15) e.g. a master cylinder to a vehicle brake (32) is modulated in accordance with skid signals from skid sensing means suitably an opto-sensor (5, 6) which signals are electrical in nature, and an hydraulic pump (2), e.g. wheel-driven, incorporating at least one plunger (10) working in a bore (11) has a working chamber (23) to control brake re- application following skid correction. Fluid from the supply (15) to the brake is modulated by a solenoid-operated valve assembly (3) which is responsive to the skid signals, the valve assembly incorporating a normally-open isolating valve (47) disposed between the supply and the brake, and a normally closed dump valve (46) disposed between the brake (32) and a reservoir (35) for fluid. An arrangement employing twin pumps for a dual circuit systems disclosed. An alternative construction of the valves 46, 47 is also disclosed. <IMAGE>

Description

SPECIFICATION Improvements in hydraulic anti-skid braking systems for vehicles This invention relatesto hydraulic anti-skid braking systems for vehicles, for example motor cycles or light passenger cars, ofthe kind in which a supply of operating fluid from a supply to a vehicle brake is modulated in accordance with skid signals from skid sensing means, and an hydraulic pump incorporating at least one plunger working in a bore has a working chamber to control brake re-application following skid correction.

In some known anti-skid braking systems of the kind setforth the supply of fluid from the supplyto the brake is modulated by a modulator assembly which comprise a piston, and an isolating valve which is operable by the piston, and a dump valve is normally closed to isolate the piston from a reservoirforfluid and trap a volume offluid between the pump andthe piston in orderto hold the piston in an advanced position in which the isolating valve is open, the dump valve opening in response to a skid signal to release the trapped volume of fluid to the reservoirthereby permitting the piston to move into a retracted position to close the isolating valve and relieve the pressure of fluid applied to the brake.In such a known construction the skid sensing means may comprise a flywheel mechanism which is operable to open the dump valve by means of a mechanical connection.

Flywheel assemblies and associated modulator assemblies are relatively bulky and include a considerable number of components. This makes installation in a limited space difficult, and assemblytime is relatively long in view ofthe number of components involved.

According to our invention in an anti-skid braking system ofthe kind setforth the skid signals are electrical in nature, and the supply offluid from the supplytothebrake is modulated bya solenoidoperated valve assembly which is responsive to the skid signals, the valve assembly incorporating a normally-open isolating valve disposed between the source and the brake, and a normally closed dump valve disposed between the brake and a reservoir for fluid.

The skid sensing means may comprise an optosensor and a remote controller operative to energise the solenoid in the event of a skid occuring whereafter the isolating valve is adapted to close to isolate the source from the brake and the dump valve is adapted to open to connect the brake to the reservoir, fluid, in a known manner, then being pumped in a closed circuit until the dump valve closes again following correction ofthe skid which permits re-application ofthe brake at a controlled rate until the isolating valve re-opens to re-connectthesource to the brake.

Preferably the dump valve is normally held in a closed position by a spring, and the isolating valve is held open against its return spring, by means of a stronger, caged, spring.

Our invention provides a simplified construction in which the relatively long travel ofthe armature ofthe solenoid-operated valve is utilised to perform the dual task of opening the dump valve and of closing the isolating valve.

In one construction a single plunger working in a bore in a housing is urged in one direction buy a cam on a drive shaft, and the working chamber controls brake re-application following skid correction and the restoration ofthe solenoid-operated valve assembly to its initial position atthe termination ofthe skid signal. In such a construction the working chamber may control re-application of the brakes on one or more wheels of a vehicle in accordance with operation ofthe solenoidoperated valve assembly in response to skid signals from the, or one, braked wheel.

In another construction the drive shaft is provided with axially spaced cams to act on two or more separate pistons, and the pistons are associated with individual working chambers, each of which is adapted to control re-application of different brakes on one or more wheels ofthe vehicle in accordance with operation of individual corresponding solenoidoperated valve assemblies, each responsive to skid signalsfromthe, or one, corresponding braked wheel.

When two cams are provided, conveniently the cams are relatively displaced angularly by 180" a round the common shaft.

Providing separate assemblies of pumps and solenoid-operated valves enables us to control separately the behaviourofdifferentwheels of a wheel, for example a front wheel and a diagonally opposite rear wheel, in accordance with skid signals from one of the wheels.

The two assemblies may be accommodated within a single common housing.

Some embodiments odour invention are illustrated in the accompanying drawings in which: Figure lisa section through a pump and skid sensing assembly for a vehicle hydraulic anti-skid braking system; Figure 2 is a section through a seal carried by the plungerofthe pump; Figure3 is an end view of a modified pump and skid sensing assembly; Figure 4 is a section on the line 4-4 of Figure 3; Figure 5is a section on the line 5-5, Figure 6 is a section similarto Figure 1 of another dump and skid sensing assembly; and Figure 7is a section of a portion of the assembly of Figure 6 but showing a modification.

The assembly illustrated in the drawings comprises a housing 1 incorporating an hydraulic pump 2, and a solenoid-operated valve assembly 3. A longitudinally extending shaft 4 projects at opposite ends from the housing and atone end carries a toothed discS forming part of an opto-sensor 6 carried by the housing 1. The shaft4 is journalled for rotation in spaced bearings 7 and 8 in the housing 1 and is driven from a front driven wheel of a vehicle.

The pump 2 comprises a plunger 10 of stepped outline which works in a stepped bore 11 in the housing 1 andtheinnerendofsmallerarea is engageable with a drive mechanism comprising a ring 12 which isfreely rotatable on an eccentric portion of the shaft 4. At its end of greater area the plunger 10 is adapted to be engaged by a drive piston 13 working in the bore of sleeve 14 of cup-shaped outline which acts as a closureforthe end ofthe bore 11 which is of greater area. The end ofthe drive piston 13 remote from the plunger 10 is exposed to the pressure in an hydraulic master cylinder 15 through a passage in the sleeve 14. The master cylinder 15 is of conventional hydrostatic construction and has its own reservoir (not shown) to replenish the pressure space in advance of its piston through a recuperation valve.

The inner end portion ofthe plunger 10, which is of smaller area, carries an annular sealing ring 16 backed buy a seal 17, an '0' ring seal 18 and a lip seal 19. The seals 18 and 19 are disposed on opposite sides of a passage 20 leading to the solenoid-operated valve 3, irrespective ofthe position ofthe plunger 10 within the bore 11, and the seal 16 and the adjacent '0' ring seal 18 are also disposed on opposite sides of a passage 21 leading to the solenoid-operated valve 3. A secondary working chamber 22 is defined by the annular volume bounded bythe bore 1 1,the plunger 10, the seal 18 and the seal 19.

A primary working chamber 23 is defined in the plunger10 between a first one-way valve 24 in the plunger 10, the drive piston 13, and a second one-way valve constituted by the seal 19.

The seal 19, which is illustrated in detail in Figure 2, comprises an annular ring of elastomeric material which is received in an annular groove 25 in the plunger 10. The groove 25 is parallel sided and is of an axial length greaterthan thethicknessofthe ring 19.

Opposite faces 26 and 27 ofthe ring are respectively plantar and of reduced area, with the face 27 of reduced area being provided with at least one diametrical slot 28 which communicates with passages 29 in the inner peripheral edge ofthe ring 19. The planarface 26 is adapted to seal against the adjacent, inner, face of the groove 25 to prevent flow from the secondary chamber 22 into the primarychamber23.

The passage 21 leads into an annular outlet chamber 30 defined between the steps in diameters of the plunger 10 and the bore 11, between the seals 16 and 18. The chamber 30 is in open communication with the working chamber 23 through connecting passages 31 in the plunger 10, and the chamber 30 is also connected to a brake 32.

The solenoid-operated valve assembly3 comprises a stepped body 40 which is received at its inner end in a stepped bore 41 in the housing 1. coil 42 which is adapted to be energised by electrical signals generated by a controller in response to skid signals from the opto-sensor 6, is housed in the body 40 and an armature 43 movable axially in response to energisation ofthe coil 41 is normally urged inwardly by means of a spring 44 which abuts against a plug 45 at the outer end ofthe body 40. The armature 43 is adapted to control operation of a dump valve 46 and of an isolating valve 47.

The dump valve 46 comprises a valve head 48 which is guided to slide in a bore 49 in the body 40. The armature 43 acts on one end ofthe head 48 normally to urge the opposite end into engagement with a seating 50 to isolate the brake 32 from a reservoir 35 by closing offthe passage 20. In this position the head 48 acts through a reduced diameterstrut51, passing through the seating 50, and an enlarged head 52 in orderto hold the isolating valve 47 in an open position so that the master cylinder 15 is in communication with the brake 32 th rough a chamber 34, to which the drive piston 13 is exposed, and the passage 21.

The isolating valve 47 comprises a head 53 for engagementwith a seating 54 defined byafaceatthe inner end ofthe body 40, and the head 53 is supported buy a stem 55 which is guided to slide in a bore in the head 52 to form a lost-motion connection therebetween. Normally, in the position shown,the head 53 is held away from the seating 54 by means of a caged spring 56 which abuts between adjacent ends ofthe heads 52 and 53 and acts against the force in a weaker return spring 57.

In a normal inoperative "brakes-off" position shown in the drawing the armature 43 is urged into an advanced position bythe spring 44to hold the dump valve 46 closed and the isolating valve 47 open. The plunger 10 is retained in an inoperative, retracted, position by means of a spring 1 Oa.

When the brake 32 is to be applied fluid from the master cylinder 15 is supplied to the brake 32 through the chamber 34, the open isolating valve 47, the passage 21 and the chamber 30.

The pressure also acts againstthe drive piston 13 to tend to urge it, and the pump plunger 10, towards the ring 12. Howeverthis is opposed bythefluid pressure in the chamber30 acting over a shoulder at the step in diameter of the piston which isofgreaterareathan that ofthe drive piston 13. Thus the drive piston 13 is held against a stop defined by the inner end of the sleeve 14 and the pump remains stalled or disabled.

If the angulardeceleration ofthe braked wheel exceeds a predetermined value, a skid signal is produced and the coil 42 is energised to withdraw the armature 43 againstthe force in the spring 44. In sequencethis opens the dump valve 46to connectthe brake 32 to the reservoir 35, and permits the isolating valve 47 to close underthe load in the spring 57. This results in the reduction of the brake-applying pressure, and the isolation ofthe master cylinder 15from the brake 32, respectively.

The release offluid-pressure from the chamber 30 causes the plunger 10 to become unbalanced and the pressure from the master cylinder 15 is sufficient to drive the drive piston 13 and the plunger 10 relatively towards the ring 12. Thus the plunger 10 is reciprocated in the bore 11 bythe ring 12which, in turn,is osciilated by rotation ofthe shaft 4.

Asthe plunger lOis movedtowardsthe ring 12, fluid from the reservoir35 is drawn into the increasing volume ofthe chamber 22 pastthe seal 19 with flow taking place th rough the passages 28 and 29, and fluid is pumped from the chamber 30 into the passage 21, since the first one-way valve 24 is closed, and from whence it is returned to the reservoir 35 through the still open dumpvalve46.Asthe piungerl0 is moved in the opposite direction the face 26 ofthe seal 19 seals against the adjacentface ofthe groove 25 so thatfluid is displaced from the chamber 22, and into the chamber 23, through the first one-way valve 24, which opens.

Whilstthe dump valve 46 remains open fluid is pumped bythe pump 2 in a closed circuitfrom the reservoir 35, to which it is returned.

Atthetermination ofthe skid signal,the spring 44 urges the armature axially to close the dump valve 46 thereby isolating the pump 2 and the brake 32 from the reservoir 35. The isolating valve 47 remains shut and is prevented from opening by the pressure from the master cylinder 1 Swhich acts to hold the head 53 againstthe seating 54, with movement ofthe armature43 relativetothe head 47 being accommodated by the lost-motion connection comprising the clearance between the stem 55 and the bore in the head 52.

The pump 2 continues to operate and, due to closure ofthe dump valve 46, the pressure from the pump 2 increases and that pressure is applied to the brake 32 at a controlled rate until that pressure either causes another skid or becomes substantially equal to the outputpressurefromthe mastercylinder 15.

In theformer case the cycle described above will be repeated to relieve the pressure applied to the brake 32 until a similar stage in the cycle is again attained.

In the lattercase,when substantially equal pressures are applied to opposite sides ofthe head 53, since the spring 56 is strongerthanthe spring 57,the isolating valve 47 will open to re-connectthe master cylinder 15 to the brake 32. The pump 2 is again stalled atT.D.C. or disabled as described above, with the plunger 10 being held awayfrom the ring 12 bythe fluid-pressure acting overthe annular area ofthe chamber 30.

When the brake is released with the isolating valve 47 in a closed position, the isolating valve 47 will open substantially upon release ofthe pressure from the master cylinder 15 and the spring 1 Oa will return the piston 10 to its stop.

In the construction described above the isolating valve47 remains closed andasourceseparatefrom the master cylinder 15, namelythe pump 2, is cycled to re-applythe brake 32 at a controlled rate.

The system described above may be provided with a "hold" feature to ensure that the dump valve 46 can open onlywhen a skid is genuine.

This can be achieved by giving the solenoid a cycle of 1 Omsto permit the isolating valve 47 to close but preventafull reduction in brake pressure. Once this cycle has been completed the brake 32 is re-applied at a rate determined by the pump 2. Ifthe skid signal persists indicating that the skid is genuine then the dump valve 46 re-opens to relieve the brake-pressure.

In the assembly illustrated in Figures 3-5, a second pump 60 is housed in thehousing 1 in a position spaced axiallyfrom the pump 2. The shaft4 is provided with a second drive mechanism comprising a ring 61 freely rotatably on a second eccentric portion on the shaft which is displaced angularly by 180 from the other eccentric portion. The two pumps 2 and 60 are similar in construction and reference numerals have therefore not been applied to the pump 60 for ciarity.

A second solenoid-operated valve assembly 62 similarto the assembly 3 is provided for association with the pump 60.

The pressure from the master cylinder 15 acts on both pistons 13, the reservoir 35 is connected to both pumps 2and 60, andtheseparate outletchambers30 are connected to the brakes 63,64 on different wheels of a vehicle.

Although we illustrate both pistons 13 being exposed to pressurefrom a common pressure space of a master cylinder 15, in a modification the pistons 13 could be exposed to pressure from different pressure sources of a tandem or dual master cylinder.

The opto-sensor 6 normally senses the behaviour of the wheel from which the shaft4 is driven, for example a front wheel of a front wheel driven vehicle, and the skid signal from that sensor operates the solenoidoperated valve 3. In another construction the optosensor 6 could simplywarn of failure in the drive to the assembly, and need not operate the solenoid. In such a case the solenoid will be operated from an independent wheel speed sensor.

The solenoid-operated valve assembly 62 is operated byasignalfrom anotherwheel atthevehicle, suitably the diagonally opposite rearwheel.

When skid signals are received from either sensor the respective solenoid-operated valve assembly 3 or 62 is operated as described above witch reference to Figures 1 and 2 to control the behaviour ofthe wheel, independently ofthe behaviour ofthe other wheel included in that brake-applying circuit.

The shaft 4 may be driven from a front wheel of the vehicle, orfrom the output shaftfrom the gear-box.

If a vehicle is aqua-planing atfastspeed on a very wet surface, when the shaft 4 is driven from a front wheel,thefurtherre-application of brake onthe rear wheel will be delayed until the wheel has passed onto a good surface so that rotation ofthefrontwheel can re-commence.

In the assembly described above with reference to Figures 3-5, the one-way valve 24 in the piston of each pump 2,60 is replaced by a second seal 65 in back-to-back relationship with the seal 18 to permit fluid to be displaced to the respective brake 63,64 as the respective piston is urged inwardly by the respective drive 12,61. The '0' ring seal 18 is replaced by a seal 66 which is subjected on opposite faces to the output pressure to the brake and from the reservoir.

The construction and operation ofthe assembly of Figures 3-5 is otherwise the same asthat of Figures 1 and 2 and corresponding reference numerals have been applied to corresponding parts.

In another construction the shaft may be provided with more than two cams each for operating a different piston, and the cams are equally spaced angularly with respectto the axis ofthe shaft.

In the assembly illustrated in Figure 6 of the drawings the pump 2 comprises an imperforatetappet 70 of stepped outline which works in a stepped bore 71 in the housing 1 and the inner end ofthetappet79, which is of smaller area, projects through a seal 72 adjacentto the inner end ofthe bore 71 forengagement with the ring 12. A spring 73 acts normallyto bias the tappet 70 away from the ring 12. At its end of greaterareathetappet 10 is adapted to be engaged by a pump plunger 74 of stepped outline. The plunger 74 works in a stepped bore defined by a portion ofthe bore 71 which is of greater diameter, and a blind bore 75 of a sleeve 76 of cup-shaped outline which acts as a closureforthatend ofthe bore71. The end ofthe plunger74which is of smaller area is exposed to the pressure in the master cylinder 15through a passage 77 in the housing and a passage 78 in the sleeve 76.

The seal 72 and a seal 79 carried by the tappet 70 are disposed at all times on opposite sides of diametrically opposed passages 80 and 81 leading respectively from the passage 78 through a flow control valve 82 in the housing 1 and to the solenoid-operated valve assembly 3. The seal 79 and a seal 83 carried by the plunger74 are disposed at all times on opposite sides of a passage 84 which also leads to the solenoidoperated valve 3.

Theflowcontrol valve 82 is constructed and arranged to increase restriction with increase in the pressure drop acting across it. Conveniently the flow control valve may be of the kind covered by GB-A2045372.

The seal 83 is of similar construction and acts ion a similar mannerto that illustrated in Figure 2 and described above. That is to say the seal 83 acts as a one-way valve to permitfiuid to be drawn from the passage84and into a primaryworking chamber85 defined in the bore 71 between the seal 83 and the sleeve 76 upon movement ofthe plunger74 away from the sleeve 76, from whence it is displaced, through a one-way valve 86 in the plunger 74, into the passage 78 upon movement ofthe plunger 74 in the opposite direction.

The dump valve 46 comprises a head 89 at the free end of a stem 90, which is guided to slide in a bore in the armature 43, and the head 89 is engagable with a seating 91 in the housing 1 to isolate the passage 4 from a reservoir92forfluid.

The isolating valve 47 comprises a valve member 93 which is urged by a spring 94towards a seating 95 disposed between the passage 77 and an outlet passage 96 leading to the brake 32 and in open communication at all times with the passage 81. The valve member 93 is normally urged away from the seating 95 bya probe 77 on an operating member97 which rocks abut a fulcrum 99 and upon which acts a compression spring 100 to urge it against a stop abutment 101 on the valve head 89.

The reservoir 92 comprises a dump chamber in the housing 1 defined between the inner end of a bore 102 and a hollow piston 103 which is urged towards the inner end ofthe bore 102 by means of a light spring 104.

In a normal inoperative "brakes-off" position shown in the drawing the armature43 is urged intoan advanced position by the spring 44to hold the dump valve 46 closed, and the spring 100 acts on the member 98to holditagainsttheabutmentl0l,in which position the isolating valve is held open. The plunger 74 and the tappet 70 are held in inoperative, retracted positions by the spring 73 and the friction in the seals 72,79 and 83.

When the brake 32 is to be applied, fluid from the master cylinder 15 is supplied to the brake 32 through the passage 77 and the open isolating valve 47. The pressure also acts on the end ofthe plunger 74, which is of smaller area, to tend to urge it, and the tappet 70, towards the ring 12. However this is opposed by the pressure applied to the brake 32 and in the passage 81 which acts over the shoulder at the step in diameter of the tappet 70. Since this is of a greater area thatthe end ofthe plunger 74 which is of smaller area, the plunger 74 is held against a stop defined by the inner end ofthe sleeve 76 and the pump 2 remains stable or disabled.

Ifthe angular deceleration ofthe braked wheel exceeds a predetermined value, a skid signal is produced and the coil 42 is energised to withdraw the armature 43 against the force in the spring 44. In sequence this withdraws the head 89 from the seating 91 to open the dump valve 46. During this movement of the head 89, the member 98 rocks about its fulcrum 99 subsequently to permitthe valve member 93 to engage with the seating 95, thereby permitting the isolating valve 47 to close underthe load in the spring 94. This results in the reduction ofthe brake-applying pressure by the release offluid from the brake 32 to the reservoir 92, and the isolation of the master cylinder 15 from the brake 32, respectively.

Opening the dump valve 46 also causesthe release offluid from the passage 81 which causes a substantial pressure drop across the flow control valve 82 thereby restrictingflowthrough it, and causes the tappet70 and the plunger 74to become unbalanced.

The pressure from the master cylinder 15 can then urge the plunger74 and the tappet 70 relatively away from the sleeve 76to cause the tappet 70 to engage the ring 12. Thetappet70and the plunger74are both reciprocated in their respective bores by the ring 12.

Whilstthe dump valve 46 remains open and as the plunger 74 is reciprocated, fluid is withdrawn from the reservoir 92 and is pumped in a closed circuit back to the reservoir 92 through the passage 77.

Atthetermination of the skid signal, the spring 44 urges the armature axially to close the dump valve 46 thereby isolating the pump 2 and the brake 32 from the reservoir 92.

The isolating valve 47 remains shut and is prevented from opening by the pressure ofthe master cylinder 15 acting on the valve member93 which, together with the force in the spring 94, is greater than the force in the spring 100 which acts on the valve member 93 through the member 98.

The pump 2 continues to operate and, due to the closure ofthe dump valve 46, the pressure from the pump 2 increases and that pressure is applied to the brake through the flow control valve 82 at a controlled rate determined bythesetting of the flow control valve 82. Thus the pressure applied to the brake increases with relative increase in the pressure from the pump 2 and relative decrease in the pressure drop across the flow control valve 82. This continues until the increased pressure applied to the brake 32 either causes another skid, or becomes substantially equal to the output pressure from the master cylinder 15.

In the former case the cycle described above will be repeated to relieve the pressure applied to the brake 32 until a similar stage in the cycle is again attained.

In the latter case, when substantially equal pressures are applied to opposite faces ofthe valve member 93, since the spring 100 is slightly stronger than the spring 94, the isolating valve 47 will open to re connectthe master cylinder 15to the brake 32. The pump 2 is again stalled or disabled as described above.

The provision ofthe reservoir 92 as a dump or expansion chamber ensures that fluid cannot be exhausted from the system as may occurin the construction of Figures 1 and 2, and Figures 3 and 4, where no fluid is available to replenish the master cylinder 15. For example, in the constructions of Figures 1 and 2, and Figures 3 and 4, which include an "atmospheric" reservoir 32, continuous cycling of the master cylinderto achieve intermittent brake application and release, generally known as "cadence braking" may exhaust all the fluid from the reservoir of the master cylinder 15. If the pump 2 is stalled, at that stage,fluid in the reservoir 35 is not available to replenish the master cylinder 15.

In the assembly of Figure 6 the piston 103 prevents fluid from escaping from the system so that said fluid is always available to replenish the pressure space of the mastercylinder 15 if necessary.

The construction and operation of the assembly of Figure 6 is otherwise the same as those of Figures 1 and 2, and nd Fig u and Figures3and4,andcorresponding reference numerals have been applied to corresponding parts.

Figure 7 shows a modified construction of the dump valve 46 and the isolating valve 47.

As illustrated both valves 46 and 47 are spaced radially with respect to the axis of stem 90, and the stem 90 carries at its free end a cross-bar 110.

The dump valve 46 comprises a head 111 which is guided to slide in a bore 112 for engagement with the seating 91, and the head 111 is carried by a stem 113 which projects th rough an opening in the bar 110 and ofwhich the free end is carried a head 115.

The isolating valve 47 comprises a valve member 116 which is urged bythe spring 94towards the seating 95. The valve member 116 is normally urged away from the seating 95 by a probe 117 which extendsfrom one side of a head 118through an opening in the bar 110, and a projection 119 projects from the opposite side of the head 118 towards the armature 43. The spring 100 acts on the heads 115 and 118through a plate 120, and the projection 119 extends through an opening in the bar 120 to locate the probe 117 in position.

In the position shown the spring 100 holds the head 111 against the seating 91, and the probe 117 holds the valve member 116 away from the seating 95. The clearance x betweenthe head 115 andthe bar 110 is greaterthan the distance "y" comprising the clearance between the valve member 116 and the seating 95. When the solenoid is energised in response to a skid signal and the stem 90 is withdrawn, the probe 117 is withdrawn through a sufficient distance "y" for the valve member 116 to engage with the seating 95 beforethebarl 10 can movethrough thedistance"x" to engage with the head 115. This ensures that the isolating valve 47 closes before the dump valve 46 can open.

Claims (20)

1. An anti-skid braking system ofthe kind set forth in which the skid signals are electrical in nature, and the supply offluid from the supply to the brake is modulated by a solenoid-operated valve assembly which is responsive to the skid signals, the valve assembly incorporating a normally-open isolating valve disposed between the source and the brake, and a normally closed dump valve disposed between the brake and a reservoirforfluid.

2. A system as claimed in claim 1, in which means responsive to a skid signal is operative to energise the solenoid in the event of a skid occurring whereafter the isolating valve is adapted to close to isolate the source from the brake and the dump valve is adapted to open to connect the brake to the reservoir, fluid then being pumped in a closed circuit until the dump valve closes again following correction of the skid which permits re-application of the brake at a controlled rate until the isolating valve re-opensto re-connectthesourceto the brake.

3. A system as claimed in claim 2, including a flow control valve th rough which fluid is pumped to re-apply the brake and which determines the control- led rate of brake re-application.

4. Asystem as claimed in any preceding claim in which the skid sensing means comprises an optosensor and a remote controller operative to energise the solenoid.

5. A system as claimed in any preceding claim, in which the dump valve is normally held in a closed position by a spring, and the isolating valve is held open against a return spring, by means of a second, stronger, spring.

6. A system as claimed in claim S, in which the dump and isolating valves comprise valve members which co-operate, sequentially, with respective valve seating in response to operation ofthe solenoid.

7. A system as claimed in claim 6, in which the valve members are aligned axially and a lost-motion connection is provided between the two members.

8. A system as claimed in claim 6, in which the valve members are relatively spaced radially and are operated by a rocking lever.

9. Asystem as claimed in claim 8, in which both valve members are spaced radially with respectto the axis of the solenoid.

10. A system as claimed in any preceding claim in which a single plunger working in a bore in a housing is urged in one direction by a cam on a drive shaft, and the working chamber controls brake re-application following skid correction and the restoration ofthe solenoid-operated valve assemblyto its initial position atthetermination ofthe skid signal.

11. Asystem as claimed in claim 10, in which the working chamber controls re-application ofthe brakes on one or more wheels of a vehicle in accordance with operation ofthesolenoid-operated valve assembly in response to skid signals from the, or one, braked wheel.

12. Asystem as claimed in any of claims 1 to 9, in which a drive shaft is provided with axially spaced cams to act on two or more separate pistons, and the pistons are associated with individual working chambers, each of which is adapted to control re-application of different brakes on one or more wheels ofthe vehicle in accordance with operation of individual corresponding solenoid-operated valve assemblies, each responsive to skid signals from the, or one, corresponding braked wheel.

13. Asystem as claimed in claim 12, in which the cams are relatively displaced angularly by 1 80C around the common shaft.

14. A system as claimed in claim 12, in which separate assemblies ofthe pumps and the solenoidoperated valves are accommodated within a single common housing.

15. Asystem as claimed in any preceding claim, in which the reservoir comprises an atmospheric reservoir.

16. Asystem as claimed in any of claims 1 to lOin which the reservoir comprises a dump or expansion chamber defined between a bore and a piston which is biassed buy a light spring.

17. An anti-skid braking system substantially as described herein with reference to and as illustrated in Figures 1 to 3 ofthe accompanying drawings.

18. An anti-skid braking system substantially as described herein with reference to and as illustrated in Figures4to 5 ofthe accompanying drawings.

19. An anti-skid braking system substantially as described herein with reference to and as illustrated in Figure 6 ofthe accompanying drawings.

20. An anti-skid braking system substantially as described herein with reference to and as illustrated in Figures 6 as modified by Figure 7 of the accompanying drawings.