GB2111620A

GB2111620A - Deceleration-sensing modulator valve assembly for vehicle braking systems

Info

Publication number: GB2111620A
Application number: GB08136310A
Authority: GB
Inventors: Ivan Mortimer
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1981-12-02
Filing date: 1981-12-02
Publication date: 1983-07-06
Also published as: IN156198B; ES8400950A1; DE3244481A1; FR2517270B1; FR2517270A1; GB2111620B; ES517810A0; JPS58110351A

Abstract

In one construction, Figure 1, a deceleration sensing ball (17) is housed in a cage (18) which is resiliently biassed towards a step (19) in the housing (1) and towards a shoulder (23) on a control piston (9) by a spring (20). A valve seat (15) is provided on the control piston at one end of a piston passage (16) that normally connects the inlet port (7) connected to a master cylinder with outlet port (8) connected to the rear wheel brakes. A pressure effective area (A1) of the control piston normally subject to inlet pressure is slightly less than an oppositely acting pressure effective area (A2) subject at all times to outlet pressure. When the ball (17) engages with the valve seat (15) due to excessive deceleration a further rise in inlet pressure urges the piston towards the outlet, and the ball is unseated from the seat (15) by a lip (22) on the cage, to produce a step in the output characteristic. An arrangement (Figure 4) for use with a dual circuit system, one circuit supplying the front wheels and the other circuit supplying the rear wheels through the modulator valve assembly. <IMAGE>

Description

SPECIFICATION Deceleration-sensing modulator valve assembly for vehicle braking systems This invention relates to deceleration-sensing modulator valve assemblies for vehicle braking systems of the kind comprising a housing provided with a brake pressure inlet and a brake pressure outlet, a control piston working in the housing which controls the relationship between the inlet and outlet pressures, and a deceleration sensing element located in the housing and adapted to operate a deceleration valve for decelerations of the housing above a deceleration threshold, operation of the deceleration valve being arranged to influence the control exerted by the control piston. Such a valve assembly will hereinafter be referred to as 'a valve of the kind set forth'.

There have been many proposals for valve assemblies of this kind. Usually the brake pressure inlet of such an assembly is connected to a brake pressure supply which is also connected to the front wheel brakes, the brake pressure outlet is connected to the rear wheel brakes, and the arrangement of the control piston is such that for inlet pressures above an inlet pressure threshold value the rate of increase of the outlet pressure is reduced compared with the rate of increase of the inlet pressure, in order to provide a proportionately reduced pressure to the rear wheel brakes to take account of the changes in weight distribution during deceleration of the vehicle.

It is known to arrange the deceleration valve in various ways in order to modify the relationship between inlet and outlet pressures. The deceleration valve is provided to take account of the fact that a larger brake pressure is required to produce a given deceleration in a loaded vehicle than for the unloaded condition of the vehicle, and it has therefore been proposed to modify the inlet pressure threshold value by making the threshold value dependent upon the closure of the deceleration valve. This avoids the use of a mechanical linkage which is deflected on a change of loading of the vehicle.

In one type of such valve assemblies closure of the deceleration valve is arranged to trap a volume of fluid in an adjustment chamber bounded by an adjustment piston which controls the spring biassing of the control piston. Usually a control spring for the control piston acts between the two pistons but such an arrangement leads to a relatively long valve assembly and its operating characteristics are not entirely satisfactory.

Patent Specification G.B. No. 2010425A shows one attempt to provide better characteristics, but this has resulted in an assembly comprising many parts.

One aim of the present invention is to provide a valve assembly of the kind set forth which is compact and relatively cheap to manufacture yet which may be arranged to have satisfactory operating characteristics.

According to the invention in a valve of the kind set forth the deceleration sensing element is housed in a cage which is slidable within a chamber subject to inlet pressure and is resiliently biassed towards engagement with a first abutment on the housing and a second abutment on the control piston, the deceleration valve comprises a valve seat on the piston which is adapted to be closed against communication with the inlet by the deceleration sensing element, the cage comprises an unseating portion engageable with the deceleration sensing element and arranged such that the valve seat is opened on relative movement apart of the piston and cage from the relative position in which the cage is engaged with the second abutment, and the piston has a first pressure effective area exposed to inlet pressure to produce a force on the piston acting in the direction away from the cage, and a second, oppositely acting pressure effective area exposed to outlet pressure, the second area being larger than the first area, the area of the valve seat being included in the computation of the first area.

With such a valve a substantial discontinuity in the characteristic of output pressure versus input pressure may be produced, subsequent to closure of the valve seat by the deceleration sensing means, as a result of engagement of the cage with the first abutment which thereby relieves the control piston of the spring force. The loss of spring force on the control piston is one important factor which necessitates a substantial rise in inlet pressure before the outlet pressure increases again. Such a discontinuity is useful in displacing the characteristic away from the theoretically ideal curve which would otherwise be intersected by a subsequent metering portion of the characteristic and would result in a locked condition of the rear wheels.

Also, with such a valve it is possible to make the first and second areas substantially the same, so that with a loaded vehicle an amplification portion of the characteristic which commences on closure of the valve seat by the deceleration sensing element, and which has a slope corresponding to the ratio of the two areas, can be arranged to have a slope of almost 4EO, which means that the theoretical curve can be followed closely at low input pressures.

Preferably the first abutment is provided by a step in the housing which is engaged by one end of the cage.

Preferably the unseating portion of the cage is provided by an annular radially inwardly directed lip on the cage, and said lip is engageable with the second abutment on the piston to determine the extent of movement together of the piston and cage.

The resilient means biassing the cage preferably comprises a compression spring which bears on an endwall of the cage at its end remote from the piston.

A direct sealing engagement may be provided between the deceleration sensing element and the valve seat, but aiternatively an apertured endwall of the cage adjacent to the piston may be arranged to seal around the aperture with the valve seat on the one hand and with the deceleration sensing element on the other hand to provide an indirect seal between the deceleration sensing element and the valve seat.

In an embodiment for use with a dual circuit braking system the control piston is formed in two normally abutting parts, a first piston part provided with said valve seat and exposed at the end remote from the valve seat to the outlet pressure of one circuit, and at its opposite end to the inlet pressure of that circuit, and a second piston part exposed at the end remote from the first piston part to the unmodified master cylinder pressure in the other circuit. In the event of fluid pressure failure of said other circuit the first piston part is disabled, so that the outlet pressure to said one circuit is not reduced over the inlet pressure.

The invention will now be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a longitudinal cross-section of a valve assembly in accordance with the invention with the parts being shown in the unactuated condition of the assembly; Figure 2 is a graph illustrating the operation of the assembly of Figure 1; Figure 3 shows a modified sealing arrangement to that employed in the Figure 1 embodiment but on a larger scale; Figure 4 is a longitudinal cross-section of a valve assembly suitable for use with a dual circuit braking system and in accordance with the invention, the parts being shown in the unactuated condition of the assembly; and Figure 5 is a graph illustrating the operation of the assembly of Figure 4.

With reference to Figure 1, the assembly comprises a housing 1 provided with a stepped bore 2 comprising bore portions 3, 4 and 5 of progressively smaller diameter. The bore 2 is closed at one end by a plug 6 provided with an inlet port 7 for connection to a master cylinder outlet and the opposite end of the bore communicates with an outlet port 8 for connection to a rear wheel brake actuator. A stepped control piston 9 has its larger diameter left hand part 10 slidably sealed in bore portion 5 by an O-ring seal 11 carried in an external annular recess of the piston, and its smaller diameter, right hand part 12 is siidable through a stationary ring 13 and is sealed to the ring 13 by an adjacent O-ring 14 compressed radially between the wall of bore portion 4 and the radially outer surface of piston part 12.The ring 13 enables the bore 3 to be machined from one end whilst providing the necessary constriction complementary to piston part 12. A vent 13' to atmosphere is provided behind ring 1 3.

Piston 9 is provided with an axial through bore 1 6 which normally provides direct communication between inlet 7 and outlet 8. At its right hand end the piston part 12 is formed with a frusto-conical valve seat 1 5 surrounding the bore 16 and sealingly engageable by a deceleration sensing element in the form of a b'all 1 7 housed in a tubular cage 18. Cage 18 is axially slidable in bore portion 3 and is biassed towards a step 1 9 in the housing between bore portions 3 and 4 by a pre-loaded coiled compression spring 20 acting between plug 6 and a closure plate 21 which retains the ball 1 7 captive within cage 18.

The cage 1 8 is provided at its left hand end with a radially inwardly directed annular lip 22 which is engageable with an annular shoulder 23 provided on piston part 12 by a reduction in the external diameter of end portion 24. End portion 24 fits with substantial clearance within the central aperture defined by the lip 22, and its length is chosen such that when the cage 18 is in abutment with the step 23 the ball 17 can move into sealing engagement with seat 1 5 without being obstructed by lip 22, but that only slight relative movement apart of the cage 18 and piston 9 is needed before lip 22 engages with ball 1 7 to remove the ball 1 7 from seat 1 5.

Cage 18 is provided with at least one axial slot 25 which extends through lip 22 to ensure that inlet pressure reaches the chamber in which 0 ring 14 is located.

The operation of the assembly of Figure 1 will now be described with reference to Figure 2. The smooth curves A and B represent the theoretically ideal relationships between the rear and front wheel brake pressures for the vehicle in the driver-only and fully laden conditions respectively.

Considering firstly the operation of the assembly in the driver-only condition, on initial application of the brakes the parts will be in the positions shown in Figure 1. The housing 1 is mounted in a vehicle with the left hand end pointing forwardly of the vehicle and at a small inclination to the horizontal so that the right hand end is lower than the left hand end. Due to this inclination the ball 1 7 rests against the plate 21 well clear of valve seat 1 5 so that initial rises in inlet pressure are transmitted directly to the outlet 8 by bore 1 6 to produce the portions C to D of the graph. It will be seen from Figure 2 that the portion C to D lies wholly below the theoretical curve A. It is particulariy important at low brake pressures that the output pressure should not exceed the theoretical curve, otherwise the rear wheels may lock. At point D, which is just below curve A, it is arranged that the vehicle deceleration in the driver-only condition is just sufficient to cause ball 17 to roll in cage 18 leftwardly to engage valve seat 15, the piston 9 being held in the position shown in Figure 1 due to the rightward force generated by inlet pressure (equals outlet pressure) acting over the opposed pressure effective areas A1 and A2 of piston parts 12 and 10 respectively. Area A1 corresponds to the bore of ring 13, and A2 corresponds to the cross-sectional area of bore portion 5, and since A2 is greater than A1 the net force acting on the piston 9 acts rightwardly.

It is arranged that the net rightward fluid pressure force acting on piston 9 at point D is just equal to the pre-load of spring 20. This ensures that for a loaded condition of the vehicle, as will be explained hereafter, the cage 1 8 would have been displaced from step 1 9 at the point at which the ball 17 rolls.

When the inlet pressure in the driver-only condition is increased from point D the increasing inlet pressure acting over the area A, with the seat 1 5 closed will result in a rise in the leftward fluid pressure force on piston 9 so that it will move to the left to take up the clearance between the ball 17 and the lip 22 to bring the ball 17 into engagement with the lip 22. Since the cage is supported by the housing by its abutment with step 1 9 engagement of the ball with the lip 22 will result in an efFective loss in the area of piston 9 over which the inlet pressure acts. This loss of area is equal to the area A3 enclosed by the line of contact of the ball 17 with the valve seat 1 5.

Since the interior of the valve seat 1 5 still communicates by way of bore 1 6 with the outlet 8, there will also be an effective loss in the area A2 over which the outlet pressure acts, to give an operative area A2-A3.

Also, on leftward movement of piston 9 from the position shown in Figure 1 the force of spring 20 is lost to the piston 9 since it is taken entirely by the housing 1 through abutment of the cage 18 with the step 19. This means that the inlet pressure must rise by an amount to compensate for the loss of spring force.

The result of the effective change of area A3 of the piston 9 and the loss of spring force is that the inlet pressure must rise by a substantial amount, to point E, before any further increase in outlet pressure takes place.

At point E the force generated by the inlet pressure acting rightwardly on piston 9 over an area A1-A3 becomes equal to the force generated by the outlet pressure acting leftwardly on piston 9 over the area A2-A3 so that on further rise in inlet pressure the piston 9 moves to the left and the ball 1 7 is unseated from valve seat 15 by the stationary lip 22. A metering action then ensues as the piston 9 shuttles backwards and forwards with the valve opening and closing to produce the portion F of Figure 2 which has a slope of (A1-A3)/(A2-A3).

Although the portion F of the curve would eventually intersect the theoretical curve A, this will only happen at high inlet pressures, at which the wheels would have locked anyway.

Considering now the operation of the assembly of Figure 1 in the fully laden condition of the vehicle, in this condition the inlet pressure will rise from C to G before a sufficient vehicle deceleration has been produced to cause rolling of ball 1 7 to engage seat 15. At point D the net rightward force due to inlet pressure acting rightwardly on piston 9 over an area of A2-A1 was arranged to equal the pre-load of spring 20 so that during the inlet pressure rise D to G the piston 9 and cage 18 are moved rightwards relative to housing 1 by the increasing fluid pressure force on piston 9, thereby to increase the loading of spring 20.

Following closure of the valve seat 15 at point G by the ball 17 further rise in inlet pressure acting over area A, will result in progressive leftward displacement of piston 9 along with cage 18, so that the outlet pressure will rise at a rate determined by the area ratio A1/A2 since valve seat 1 5 is closed. This produces the portion G to H of the graph. The invention enables this ratio to be relatively close to unity to give a slope of slightly less than 45 so that a maximum amplification of the point G is produced. The theoretical curve is therefore followed closely at low inlet pressures.

At point H, which is arranged to be just below the theoretical curve B, the cage 1 8 comes into engagement with the step 1 9 and substantially.

simultaneously the area A3 of the piston 9 is rendered ineffective by leftward movement of piston 9 to bring lip 1 5 into engagement with ball 17. As in the unladen condition this results in a delay in further rise in outlet pressure as the inlet pressure rises to take account of the effective loss of area A3, and loss of spring force on piston 9 when the cage 1 8 engages housing step 1 9. At point K the leftward force on the piston 9 due to inlet pressure acting over area A1-A3 is sufficient to overcome the rightward force due to outlet pressure acting over area A2-A3, so that once again metering commences, with the same relationship between outlet and inlet pressure as for the portion F to produce the portion J of the graph, which will be seen to be effectively a continuation of the line F.

The assembly of Figure 1 therefore produces the substantial shifts D to E, and H to K of the characteristics to ensure that the actual valve characteristics do not cross the theoretical curves, except at high inlet pressures, yet the actual characteristics closely follow the theoretical curves at low pressures in the region C to D, and C to H in the driver-only and fully laden conditions respectively.

Closure plate 21 is provided with a series of orifices 26 so that on rapid application of the master cylinder due to panic braking the ball 17 is urged onto valve seat 1 5 by fluid pressure forces to displace point D towards lower pressures. The arrangement of the ball 1 7 in the direct fluid path between inlet 7 and outlet 8 facilitates this feature.

It will be appreciated that the arrangement of all of the parts in a single bore 3 rather than in separate bores assists in reducing manufacturing costs.

Figure 3 shows a modified sealing arrangement to that of Figure 1 which employs a direct metal to metal engagement between ball 1 7 and seat 1 5. Parts corresponding to those of Figure 1 have been given corresponding reference numerals in Figure 3. In arrangement of Figure 3 the cage 1 8 is provided with an end wall 27 adjacent to the piston part 12 which is provided with a central hole 28 surrounded on opposite sides of wall 27 by respective elastomeric seat rings 29, 30 bonded thereto. The seat ring 29 is adapted to engage with the chamfer 1 5 of piston part 12 to effect a seal between end wall 27 and piston part 12, and the ring 30 is sealingly engageable by the ball 17.On initial rise of inlet pressure piston 9 will move rightwardly to bring chamfer 1 5 into engagement with seat ring 29.

Then, on rolling of ball 1 7 to engage with ring 30 an indirect seal is provided between ball 1 7 and piston end 15 by way of wall 27. The radially inner edge of the chamfer 14 defines the area A3 which is rendered ineffective when the ball 1 7 is engaged with ring 30 and chamfer 1 5 is engaged with ring 29.

The valve assembly of Figure 4 is for use with a dual circuit braking system. Parts corresponding to those of Figures 1 and 3 have been given corresponding reference numerals. In this embodiment the control piston assembly comprises two separable abutting parts 31, 32. A metering valve assembly similar to that of Figure 3 is employed except that the seat ring 29 is carried by the right hand end of the piston part 32. Bore 1 6 of piston leads into an intermediate chamber 33 in the housing, by way of radial recesses 34 in the left hand end face of piston part 32, the primary outlet port 8, which is a transverse port in this case, communicating directly with chamber 33.

Inlet 7 is connected to one outlet 35 of a tandem master cylinder 36, and the second outlet 37 is connected to a port 38 leading into a chamber 39 at the lefthand end of piston part 31, that chamber communicating directly with a secondary outlet port 40. Primary port 8 is connected to a rear wheel brake actuator, not shown, and secondary outlet 40 is connected to a front wheel brake actuator, also not shown.

Piston part 32 is a plain piston exposed at its right hand end to inlet pressure and at its lefthand end to primary outlet pressure in chamber 33, whereas piston part 31 is stepped with its larger left hand end exposed to secondary inlet (equals outlet) pressure and its smaller right hand end exposed to the pressure in chamber 33. The area of the lefthand end of piston part 31 and that of its righthand end will be designated A, and A2 respectively. The cross-sectional area of chamber 33, and that of piston part 32 will be designated A3, and the cross-sectional area of seat 29 will be designated A4. (These letters do not correspond to the ones employed for Figure 1).

When both circuits are operational the characteristics of the valve assembly may be arranged to be identical to that of Figure 1, as shown in Figure 5 where corresponding parts of the characteristics have been given the same letters. The pressure effective area of the control piston assembly 31, 32 exposed to inlet pressure and acting in the direction away from the cage is (A3-A1) due to inlet pressure acting over the right hand end of piston part 32 and over the left hand end of piston part 31. The opposing pressure effective area of the control piston assembly exposed to outlet pressure is (A3-A2).

Thus, the slope of the portion G to H of the characteristic is (A3-A1)/(A3-A2), and that of the metering portions F and J is (A3-A1-A4)/(A3- A2-A4). By arranging that A, is less than A3 and by a suitable choice of the areas, identical characteristics to those of Figure 2 can be achieved.

In the event of loss of pressure in the secondary circuit connected to chamber 39, the leftward facing area A, will be lost to inlet pressure so that the control piston assembly 31, 32 will be held fully leftwardly by primary inlet pressure acting leftwardly on the area A3 on the right hand end of piston part 32, to prevent seat 29 effecting a seal with the cage wall 27. Thus, even when ball 17 engages seat 30 the intermediate chamber 33 will continue to communicate with the primary inlet 7 past open seat 29, and maximum braking will be applied to the rear wheels as indicated by the broken line L in Figure 5.

In a modification, not shown, of the valve assembly of Figure 4 piston part 31 is arranged to be a plain piston, and piston part 32 is arranged to be a stepped piston.

Claims (Filed on 18/11/82) 1. A valve of the kind set forth in which the deceleration sensing element is housed in a cage which is slidable within a chamber subject to inlet pressure and is resiiiently biassed towards engagement with a first abutment on the housing and a second abutment on the control piston, the deceleration valve comprises a valve seat on the piston which is adapted to be closed against communication with the inlet by the deceleration sensing element, the cage comprises an unseating portion engageable with the deceleration sensing element and arranged such that the valve seat is opened on relative movement apart of the piston and cage from the relative position in which the cage is engaged with the second abutment, and the piston has a first pressure effective area exposed to inlet pressure to produce a force on the piston acting in the direction away from the cage, and a second, oppositely acting pressure effective area exposed to outlet pressure, the second area being larger than the first area, the area of the valve seat being included in the computation of the first area.

2. A valve as claimed in claim 1 in which the first abutment comprises a step in the housing which is engaged by one end of the cage.

3. A valve as claimed in claim 1 or claim 2 in which the unseating portion of the cage comprises an annular radially inwardly directed lip on the cage, and said lip is engageable with the second abutment on the piston to determine the extent of movement together of the piston and cage.

4. A valve as claimed in any of the preceding claims in which the resilient means biassing the cage comprises a compression spring which bears

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. sides of wall 27 by respective elastomeric seat rings 29, 30 bonded thereto. The seat ring 29 is adapted to engage with the chamfer 1 5 of piston part 12 to effect a seal between end wall 27 and piston part 12, and the ring 30 is sealingly engageable by the ball 17. On initial rise of inlet pressure piston 9 will move rightwardly to bring chamfer 1 5 into engagement with seat ring 29. Then, on rolling of ball 1 7 to engage with ring 30 an indirect seal is provided between ball 1 7 and piston end 15 by way of wall 27. The radially inner edge of the chamfer 14 defines the area A3 which is rendered ineffective when the ball 1 7 is engaged with ring 30 and chamfer 1 5 is engaged with ring 29. The valve assembly of Figure 4 is for use with a dual circuit braking system. Parts corresponding to those of Figures 1 and 3 have been given corresponding reference numerals. In this embodiment the control piston assembly comprises two separable abutting parts 31, 32. A metering valve assembly similar to that of Figure 3 is employed except that the seat ring 29 is carried by the right hand end of the piston part 32. Bore 1 6 of piston leads into an intermediate chamber 33 in the housing, by way of radial recesses 34 in the left hand end face of piston part 32, the primary outlet port 8, which is a transverse port in this case, communicating directly with chamber 33. Inlet 7 is connected to one outlet 35 of a tandem master cylinder 36, and the second outlet 37 is connected to a port 38 leading into a chamber 39 at the lefthand end of piston part 31, that chamber communicating directly with a secondary outlet port 40. Primary port 8 is connected to a rear wheel brake actuator, not shown, and secondary outlet 40 is connected to a front wheel brake actuator, also not shown. Piston part 32 is a plain piston exposed at its right hand end to inlet pressure and at its lefthand end to primary outlet pressure in chamber 33, whereas piston part 31 is stepped with its larger left hand end exposed to secondary inlet (equals outlet) pressure and its smaller right hand end exposed to the pressure in chamber 33. The area of the lefthand end of piston part 31 and that of its righthand end will be designated A, and A2 respectively. The cross-sectional area of chamber 33, and that of piston part 32 will be designated A3, and the cross-sectional area of seat 29 will be designated A4. (These letters do not correspond to the ones employed for Figure 1). When both circuits are operational the characteristics of the valve assembly may be arranged to be identical to that of Figure 1, as shown in Figure 5 where corresponding parts of the characteristics have been given the same letters. The pressure effective area of the control piston assembly 31, 32 exposed to inlet pressure and acting in the direction away from the cage is (A3-A1) due to inlet pressure acting over the right hand end of piston part 32 and over the left hand end of piston part 31. The opposing pressure effective area of the control piston assembly exposed to outlet pressure is (A3-A2). Thus, the slope of the portion G to H of the characteristic is (A3-A1)/(A3-A2), and that of the metering portions F and J is (A3-A1-A4)/(A3- A2-A4). By arranging that A, is less than A3 and by a suitable choice of the areas, identical characteristics to those of Figure 2 can be achieved. In the event of loss of pressure in the secondary circuit connected to chamber 39, the leftward facing area A, will be lost to inlet pressure so that the control piston assembly 31, 32 will be held fully leftwardly by primary inlet pressure acting leftwardly on the area A3 on the right hand end of piston part 32, to prevent seat 29 effecting a seal with the cage wall 27. Thus, even when ball 17 engages seat 30 the intermediate chamber 33 will continue to communicate with the primary inlet 7 past open seat 29, and maximum braking will be applied to the rear wheels as indicated by the broken line L in Figure 5. In a modification, not shown, of the valve assembly of Figure 4 piston part 31 is arranged to be a plain piston, and piston part 32 is arranged to be a stepped piston. Claims (Filed on 18/11/82)

1. A valve of the kind set forth in which the deceleration sensing element is housed in a cage which is slidable within a chamber subject to inlet pressure and is resiiiently biassed towards engagement with a first abutment on the housing and a second abutment on the control piston, the deceleration valve comprises a valve seat on the piston which is adapted to be closed against communication with the inlet by the deceleration sensing element, the cage comprises an unseating portion engageable with the deceleration sensing element and arranged such that the valve seat is opened on relative movement apart of the piston and cage from the relative position in which the cage is engaged with the second abutment, and the piston has a first pressure effective area exposed to inlet pressure to produce a force on the piston acting in the direction away from the cage, and a second, oppositely acting pressure effective area exposed to outlet pressure, the second area being larger than the first area, the area of the valve seat being included in the computation of the first area.

on an endwall of the cage at its end remote from the piston.

5. A valve as claimed in any of the preceding claims in which an apertured endwall of the cage adjacent to the piston is arranged to seal around the aperture with the valve seat on the one hand and with the deceleration sensing element on the other hand to provide an indirect seal between the deceleration sensing element and the valve seat.

6. A valve as claimed in any of the preceding claims and intended for use with a dual circuit braking system, in which the control piston is formed in two normally abutting parts, a first piston part provided with said valve seat and arranged to be exposed at the end remote from the valve seat to the outlet pressure of one circuit, and at its opposite end to the inlet pressure of that circuit, and second piston part arranged to be exposed at the end remote from the first piston part to the unmodified master cylinder pressure in the other circuit.

7. A deceleration-sensing modulator valve assembly substantially as described with reference to Figures 1 and 2 of the accompanying drawings.

8. A deceleration-sensing modulator valve assembly as claimed in claim 7 but modified substantially as described with reference to Figure 3 of the accompanying drawings.

9. A deceleration-sensing modulator valve assembly substantially as described with reference to Figures 4 and 5 of the accompanying drawings.