GB2087494A

GB2087494A - Hydraulic Pressure Control Valve for Dual Circuit Braking System

Info

Publication number: GB2087494A
Application number: GB8035544A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1980-11-05
Filing date: 1980-11-05
Publication date: 1982-05-26
Also published as: FR2493782A1; FR2493782B1; DE3042125A1

Abstract

In a hydraulic pressure control valve assembly for a dual circuit braking system, which comprises two parallel proportioning valve assemblies (26,28) having a common spring (60) by which the respective plungers (30,30') are biased to control the magnitude of hydraulic pressure at the respective outlet ports (20,24) in response to application of hydraulic pressure to the respective inlet ports (18,22), and a ball-type deceleration sensing valve assembly (66) by which the magnitude of load applied to the common spring is regulated in accordance with the degree of deceleration of the vehicle, there is further provided a fail-safe valve assembly (90) which is constructed so that when the first braking system (116a) fails to operate, the pressure in the second braking system (116b) is directly applied to the outlet port (24) associated with the second braking system, but when the second braking system fails to operate, the pressure in the first braking system is allowed to compress the common spring to increase the critical pressure or split point in the first braking system at which the proportioning valve action begins. <IMAGE>

Description

SPECIFICATION Hydraulic Pressure Control Valve for Double Piping Braking System The present invention relates in general to an anti-skid braking system for a motor vehicle, and more particularly to a hydraulic pressure control valve assembly for a so-called double piping braking system which functions to control the increase from input to output hydraulic pressure therein.

A known double piping braking system comprising two braking systems is commonly used in motor vehicles due to its higher dependability. In such a system, the following two methods are usually employed: (1) a method in which the braking system is so constructed that the hydraulic pressure from one of the outlets of a tandem master cylinder is supplied to the rightfront wheel brake cylinder and the left-rear wheel brake cylinder and the pressure from the other outlet is supplied to the left-front wheel brake cylinder and the right-rear wheel brake cylinder, and (2) a method in which two brake cylinders are provided on each front wheel and the braking system is so constructed that the hydraulic pressure from one of the outlets of the mastercylinder is supplied to one of the respective right and left front wheel brake cylinders and the leftrear brake cylinder, and the pressure from the other outlet is supplied to the other of the respective right and left front wheel brake cylinders and the right-rear wheel brake cylinder.

The former method is known as X-piping and the latter as J-J-piping. In either piping method, it is necessary to provide a hydraulic pressure control valve assembly, such as a limiting valve assembly or a proportioning valve assembly, in the respective hydraulic pressure supply lines to the rear wheel brake cylinders to compensate for the effective weight transfer of the motor vehicle during braking. Therefore, the conventional double piping braking system requires two hydraulic pressure control valve assemblies. This causes disadvantages from the standpoint of mounting space and vehicle assembly.

Accordingly, there has been proposed a single valve assembly for the double piping braking system wherein a pair of proportioning valve assemblies performing the respective hydraulic pressure control functions for the rear wheel braking systems are arranged in a common housing parallel to each other, the respective plunger members of the valve assemblies being biased by a common spring acting on a common spring seat. In such proportioning valve assemblies, however, the hydraulic pressure at the beginning to slacken the increase of the rear wheel braking pressure, that is, the critical pressure, is constant, thereby rendering the distribution characteristic of the front and rear wheel braking force constant.

As is well known in the art, in order to obtain such an ideal distribution characteristic of the front and rear wheel braking force that enables the front and rear wheels to brake evenly, the braking force distribution should vary with change of vehicle weight, and the above-mentioned critical hydraulic pressure should increase with an increase in vehicle weight. In this sense, the hydraulic pressure control valve assembly of the above-mentioned type is not suitable for largesized trucks and the like, the weight of which varies greatly depending on whether they are empty or loaded.

For solving this problem, a so-called deceleration sensing valve has been proposed which is provided in the single valve assembly having two proportioning valves. With the provision of the deceleration sensing valve, the characteristic of the rear wheel brake pressure is controlled in accordance with the vehicle weight.

It is an object of the present invention to provide a hydraulic pressure control valve assembly for a double piping braking system, which assembly is provided with not only the above-mentioned deceleration sensing valve but also a fail-safe means which is constructed so that when one braking system fails to operate, the other braking system operates in such a manner that one front wheel brake cylinder and one rear wheel brake cylinder are actuated to brake the vehicle.

According to the present invention, there is provided a hydraulic pressure control valve assembly for a double piping braking system of a wheeled vehicle. The valve assembly comprises two parallel proportioning valve assemblies respectively provided with plungers which are axially movable in a direction against a common spring in response to the application of hydraulic pressure to respective inlet ports to control the magnitude of hydraulic pressure at respective outlet ports, a deceleration sensing valve assembly which, when the first and second braking systems operate normally, regulates the magnitude of load applied to the common spring in accordance with the degree of deceleration of the vehicle, and a fail-safe means which is so constructed that when the first braking system fails to operate, the fail-safe means forms a bypass passage between the inlet and outlet ports associated with the second braking system, thereby directly applying the pressure in the second braking system inlet port to the second braking system outlet port, and when the second braking system fails to operate, the fail-safe means blocks the by-pass passage and permits the pressure in the first braking system to compress the common spring, thereby increasing the critical pressure in the first braking system at which the rate of increase of the outlet port pressure of the first braking system to the inlet port pressure thereof changes.

In the accompanying drawings:- Fig. 1 is a vertical sectional view of a hydraulic pressure control valve assembly of the present invention, showing the location of the valve assembly in a motor vehicle double piping hydraulic braking system; Fig. 2 is a graph depicting the relationship between the master cylinder pressure and the rear wheel braking pressure; Fig. 3 is a graph depicting the relationship between the vehicle weight and the critical pressure; and Fig. 4 is an enlarged sectional view of a principal portion of an alternative embodiment of the present invention.

Referring to Fig. 1, there is illustrated a hydraulic pressure control valve assembly of the present invention, which is generally designated by numeral 10. The valve assembly 10 comprises a housing 12 in which a first plunger chamber 14 and a second plunger chamber 1 6 are formed.

These chambers 14 and 16 are parallel and open to an end surface 1 2a of the housing 12. In the housing 12, there are further formed a first inlet port 1 8 and a first outlet port 20 which lead to the first plunger chamber 14, and a second inlet port 22 and a second outlet port 24 which lead to the second plunger chamber 16. In the respective plunger chambers 14 and 16, there are provided first and second control valve assemblies 26 and 28 for selectively communicating and interrupting hydraulic flow between the inlet and outlet ports 18 and 20 and between the inlet and outlet ports 22 and 24.

The control valve assemblies 26 and 28 are proportioning valve assemblies of identical construction. For ease of explanation, description of the construction of these proportioning valve assemblies 26 and 28 will be directed to only one of them, for example, the assembly 26 associated with the first plunger chamber 14. But, for ease of understanding and description, parts of the other valve assembly 28 are indicated by the addition of ""'after each corresponding numeral in the drawing.

A plunger 30 is axially movably disposed in the first plunger chamber 14. The plunger 30 is formed at its left hand end with a blind hole 34 within which a poppet valve 32 is received. Fixed to the open end of the blind hole 34 by caulking is a valve seat 36, toward which the poppet valve 32 is biased by a spring 38. When the plunger 30 moves rightward as viewed in the drawing to a certain position, the poppet valve proper 32 is forced to rest on the valve seat 36 by the action of the spring 38, thereby shutting the passage between the inlet and outlet ports 18 and 20. The length of the stem part 40 of the poppet valve 32 projects from the left hand side of the valve seat 36.Thus, when the plunger 30 takes its leftmost position, the top of the stem part 40 contacts the bottom of the plunger chamber 14, thereby separating the poppet valve proper 32 from the valve seat 36 and thus permitting the valve 32 to take its open position.

The valve seat 36 has holes 42 for communication between the outlet port 20 and the interior of the valve seat 36, and the plunger 30 has holes 44 for communication between the inlet port 18 and the blind hole 34. The plunger chamber 14 tightly receives therein a sleeve 46 which surrounds a reduced diameter part of the plunger 30 with a clearance therebetween as shown. The sleeve 46 has holes 48 for communication between the inlet port 18 and the clearance. Thus, it will be appreciated that when the poppet valve 32 opens, a free passage for hydraulic fluid is formed between the inlet and outlet ports 18 and 20.

The plunger 30 projects from the open end of the plunger chamber 14, as shown. The right hand end section of the plunger 30 is guided by a retainer 50 which is coupled in the plunger chamber 14 and sealed by a seal member 52.

The projecting ends 30a and 30a' of the plungers 30 and 30' contact a flange portion 54a of a cylindrical spring holder 54. The holder 54 receives a plastics bushing 56 in a central hole 54b thereof. A guide rod 58 having a right hand section about which the bushing 56 is slidably mounted is fixed to the housing 12. A spring 60 is arranged and compressed between the flange portion 54a of the spring holder 54 and a spring seat 62 which will be described in detail hereinafter. The spring 60 thus presses the plungers 30 and 30' of the control valve assemblies 26 and 28 for maintaining the poppet valves 32 and 32' in the open position.For the reason which will become clear hereinafter, the sectional area of the central hole of the bushing 56 at the right hand section thereof gradually increases with distance from the generally middle portion of the bushing hole to form a tapered clearance 59 between the right hand section of the guide rod 58 and the right hand section of the bushing 56, as shown.

The members such as the spring 60, the spring holder 54, the bushing 56, etc. are housed within a cylindrical hollow part 64a of a housing 64 of a deceleration sensing valve assembly 66. The housing 64 is connected to the afore-mentioned housing 12 through a seal plate 68. Within the cylindrical hollow part 64a is housed another spring 70 which is arranged and compressed between the spring seat 62 and the seal plate 68 to bias the seat 62 in the rightward direction. In the housing 64 are formed a cylindrical chamber 72 opening to the interior of the cylindrical part 64a, and a ball chamber 74. The chambers 72 and 74 communicate through a passage 76 formed in the housing 64. A valve seat 78 is fixed to the open end of the passage 76 at a portion exposed to the chamber 74.

A piston 80, which is integral with the spring seat 62, is sealingly and slidably disposed in the cylindrical chamber 72 to define an expandable operating chamber section 72a between the piston 80 and the bottom of the chamber 72. An air bleed valve 82 is fixed to the housing to connect to the operating chamber section 72a. A ball 84 is movably disposed in the ball chamber 74. The ball chamber 74 is closed by a plug 86. A suitable number of grooves 88 are formed in the wall of the ball chamber 74 for communicating first and second chamber sections 74a and 74b formed at the front and rear of the ball 84.

In the hydraulic pressure control valve assembly 10 of the present invention, there is further provided a switching valve assembly 90 which acts as fail-safe means. The assembly 90 comprises a stepped cylindrical bore formed in the housing 64. The bore includes a larger diameter section 92 and a smaller diameter section 94 which are coaxially aligned. The open end of the bore is closed by a plug 96 in which an inlet port 98 is formed to communicate with the section 92. Within the bore is axially and movably disposed a piston 100. The piston 100 has a land 1 00a which is sealingly slidably received in the larger diameter section 92 to divide it into first and second chambers 92a and 92b. The chambers 92a and 92b communicate respectively with the ball chamber 74 and the atmosphere through respective passages 102 and 104 formed in the housing 64.The left-hand end of the piston 100 is sealingly and slidably received in the smaller diameter section 94 of the bore to form between the left hand end of the piston 100 and the bottom of the chamber section 94 an expandable operating chamber 94a. The chamber 94a communicates with both inlet and outlet ports 106 and 108 formed in the housing 64. A rubber ring 110 is fixed to the leftmost end of the piston 100 so that when the piston 100 takes its leftmost position, it blocks the communication between the inlet port 106 and the operating chamber 94a. For assured blocking, a valve seat 112 is mounted at a portion with which the rubber ring 110 engages.A spring 114 is disposed in the second chamber 92b of the larger diameter bore section 92 and is compressed between the piston 100 and the plug 96 to bias the piston 100 to block the communication between the inlet and outlet ports 106 and 108.

As will be understood hereinafter, in the normal condition of the braking system, the piston 100 is maintained in its leftmost position so that only the passage between the inlet port 98 and the ball chamber 74 opens.

When the hydraulic pressure control valve assembly 10 of the invention is located on the vehicle body, it is slanted at an angle "0" with respect to the vehicle body axis "0", so that under normal conditions of the assembly 10, the ball 84 is in contact with the plug 86 by its own gravity, thereby opening the passage 76 between the ball chamber 74 and the expandable operating chamber 72a.

In the practical use of the valve assembly 10, the inlet ports 98 and 106 are respectively connected to outlet ports 11 6a and 11 6b of a tandem master cylinder 11 6. The inlet port 18 is connected to the right-front wheel brake cylinder and the outlet port 11 6a of the master cylinder 11 6, while the outlet port 20 is connected to the left-rear wheel brake cylinder 120.The inlet port 22 is connected to the left-front wheel brake cylinder 1 22 and the outlet port 11 6b of the master cylinder 11 6, while the outlet port 24 is connected to the right-rear wheel brake cylinder 1 24. A brake pedal 126 actuates the tandem master cylinder 11 6 to produce identical hydraulic pressures at the outlet ports 11 6a and 11 6b thereof.

Similar to a conventional proportioning valve, each plunger 30 or 30' has a section of the diameter b1 and a section of the diameter b2 in which b1 is greater than b2.

In the following mathematical analysis, the designations, b1, b2 and b3, are used. Those skilled in the art will recognize that these designations, are to be interpreted as being the areas of the respective surfaces, b,, b2 and b3, and not the diameters.

In operation, when the brake pedal 126 is depressed to cause the master-cylinder 11 6 to produce hydraulic pressures PM1 and PM2 at the outlet ports 11 6a and 11 6b, the pressures PM1 and PM2 (in practice, PM1=PM2) are supplied to the right-front wheel brake cylinder 11 8 and the inlet port 18, and the left-front wheel brake cylinder 122 and the inlet port 22, respectively.These master-cylinder pressures PM1 and PM2 are also directly supplied to the corresponding rear wheel brake cylinders 120 and 124, respectively, because at the initial condition of the master cylinder operation, the poppet valves 32 and 32' are kept open. Thus, in this condition, the front and rear wheel braking hydraulic pressures increase with the characteristic shown by "a-b" in Fig. 2. The balance equation of the force applied to the respective plungers 30 and 30' in this condition is given as follows, in which the force generated by the spring 60 is represented by "F".

Pb2=+xF (1) (Here, PM1=PM2=PM) F . . PM= (2) 2b2 (Here, PM=PR1=PR2=PR wherein PR1 and PR2 are hydraulic pressures at the outlet ports 20 and 24, respectively).

When the master cylinder hydraulic pressure PM increases by the continuous operation of the brake pedal 126 to a degree to satisfy the following relation, PMXb2 > xF (3) the plungers 30 and 30' move rightwardly against the force of the spring 60, and the poppet valves 32 and 32' close, thereby interrupting the communication between respective inlet and outlet ports 18 and 20, 22 and 24. Thus, the pressure supply to the rear wheel brake cylinders 1 20 and 1 24 is blocked.In this condition, the following balance equation is given: PMx(b1b2)+2 xF=PRxb, (4) b1-b2 F ## PR= XPM+ (5) b, 2b, When the master cylinder pressure PM further increases due to continuous operation of the brake pedal 126, the pressure PM acting on the differential area b1-b2 causes the plungers 30 and 30' to return to their initial positions, and the poppet valves 32 and 32' open again, whereby the rear wheel braking pressures increase with a smaller slope b, - b2 b, as shown by "b-c" in Fig. 2. Thus, unwanted rear wheel skidding is prevented.

It should be noted that during the abovementioned operation, the switching valve assembly 90 keeps the position shown in Fig. 1 in which the communication between the inlet and outlet ports 106 and 108 is blocked and communication between the inlet port 98 and the ball chamber 74 is established. This is because the piston 100 is biased leftwardly by not only the force of the spring 114 but also the force corresponding to the product of the difference in pressure receiving areas between the piston land's right side end facing the chamber 92 and the piston's left hand end facing the chamber 94, and the master cylinder pressure.

To the ball chamber 74, only the master cylinder pressure from the outlet 11 6a is applied through the inlet port 98 and the passage 102 to operate the deceleration sensing valve assembly 66 in a manner described next.

As is known, when the master cylinder pressure PM increases, the braking force "B" also increases, and the deceleration "a" which is obtained by dividing the braking force "B" by the vehicle weight "W", also increases, as is clear from the following equations: B=CXPM (6) (C is a constant) a B g W (7) When the deceleration ratio "a" g reaches a predetermined value determined by the sloping angle "0" of the valve assembly 10 with respect to the vehicle axis "H", that is,

(f) 0=f(O) (8) (f(O): function of "8"), the ball 84 moves leftwardly in Fig. 1 by its weight against the component of force of the gravitational acceleration in the direction of the sloping angle "O" to close the opening of the valve seat 78.Thus, under this condition, even if the master cylinder pressure PM increases further, the pressure applied to the piston 80 is maintained at that value at the time when the ball 84 closes the opening of the valve seat 78. The pressure PG in the expandable operating chamber 72a is represented by the following equation: f(O) PG > cW (9) C The force pushing the piston 80 to the left in Fig. 1, which is represented by the product of the pressure Pc and the pressure receiving area b3 of the piston 80, balances the sum of the forces F and F' of the springs 60 and 70, and the following equation is obtained.

f(0) F+F'=PcXb3= xb3xW (10) C The force F acts to push the plungers 30 and 30' to the left in Fig. 1, and the force F' is received by the seal plate 68 interposed between the housing 12 and the housing 64.

The forces F and F' are respectively obtained by adding the values obtained by the products of the distance the piston 80 moves, Ax, and the spring constants k, and k2 of the springs 60 and 70 to the set loads f, and f2 of these springs at the time PM=O. Thus, the relation between F and F' is represented as follows: K2 F'=f2±(F-f1) (11) K, From the equations (10) and (11), the following is obtained:

On the other hand, the critical pressure Ps is represented by the equation (2) as follows: F Ps= (13) 2b2 When the equation (12) is put into this equation (13), the following is obtained::

If K2 (f2--f1) > 0 K1 is established, the relation of the critical pressure P5 with respect to the vehicle weight W is obtained as shown in Fig. 3. As is clear from this graph, the critical pressure P5 increases with an increase of the vehicle weight W.

As the split point b, shown in Fig. 2, increases with the increase of the load on the vehicle, the rear wheel braking pressure PR increases. For example, in the case of a half loading on the vehicle, the rear wheel braking pressure PR increases with the characteristic indicated by ab"-c", which is substantially the ideal, as is known to those skilled in the art.

In the event one braking system, including the brake cylinder 122 of the left-front wheel, fails to operate, the piston 100 of the switching valve assembly 90 keeps the position shown in Fig. 1, maintaining the blocking between the inlet and outlet ports 106 and 108 and keeping the communication between the inlet port 98 and the ball chamber 74. This is because the switching valve assembly 90 is constructed to maintain the illustrated position, whether or not the inlet port 106 receives pressurized fluid from the one braking system, so long as the other braking system, including the brake cylinder 118 of the right-front wheel, operates normally. Thus, even when such trouble occurs, the deceleration sensing valve assembly 66 operates normally in substantially the same manner as mentioned before. However, the proportioning valve assembly 28 does not operate.Therefore, the plunger 30' remains in the position shown in Fig.

1. When the hydraulic pressure in the normally operated braking system, which includes the right-front wheel brake cylinder 11 8, increases, the plunger 30 of the proportioning valve assembly 26 moves rightwardly against the force of the spring 60. With this, the spring holder 54 inclines gradually to pivot about the point where it contacts the plunger 30'. This inclination is permitted by the tapered clearance 59 defined between the guide rod 58 and the bushing 56.

Then, the spring holder 54, more particularly the flange portion 54a thereof, moves away from the projecting end 30a' of the plunger 30' and slides with the bushing 56 on the guide rod 58, whereby the poppet valve 32 closes. At this time, the spring 60 acts on only the proportioning valve assembly 26. Thus, the following equation applies, b, - b2 F PR P,±- (15) b, b, and thus, a larger braking force, as shown by a-b'c' in Fig. 2, is obtained by only a single braking system which at this time incorporates the rightfront wheel brake cylinder 11 8, thus compensating for an insufficiency of braking force originating as a result of the other braking system malfunctioning.

From a practical point of view, when one braking system malfunctions, the brake pedal 126 is depressed by a force twice as large as that required when both braking systems operate normally, in order to obtain the same braking force as that obtained when both systems operate normally. Thus, practically, when one braking system malfunctions, the pressure Pa confined in the expandable operating chamber 72a upon operation of the deceleration sensing valve assembly 66 shows a value twice as large as that shown when both the braking systems operate normally.Accordingly, upon strong depression of the brake pedal 126 for rapid deceleration of the vehicle under such malfunctioning, the distance the piston 80 moves,due to the increase of the confined pressure Pa in the chamber 72a (i.e., the compressed distance of the spring 60) shows a value twice as much as that shown when both braking systems operate normally, so that the rear wheel braking pressure incorporating with the proportioning valve assembly 26 increases with the characteristic indicated by a-b"'-c"' in Fig. 2. Thus, increased braking force is obtained by only a single braking system.

It should be noted that the rear wheel braking pressure indicated by the split point b"' shows a value which exceeds the value practically provided by the master cylinder. This means that upon malfunctioning of the one braking system incorporating the left-front wheel brake cylinder 122, the left-rear wheel brake cylinder 120 receives a hydraulic pressure substantially the same as that of the master cylinder pressure PM.

Thus, sufficient braking force is obtained only by the single braking system.

Likewise, when the other braking system incorporating the right-front wheel brake cylinder 11 8 fails to operate, the piston 100 of the switching valve assembly 90 moves to its rightmost position against the force of the spring 114 due to the absence of hydraulic pressure in the chamber 92, thereby opening the communication between the inlet and outlet ports 106 and 108. Thus, in this condition, the master cylinder pressure PM is directly applied to the right-rear wheel brake cylinder 124 through ports 106 and 108, unaltered by the proportioning valve assembly 28. Thus, the braking pressure in the right-rear wheel brake cylinder 124 increases with the characteristic indicated by a-d in Fig. 2.

Thus, sufficient braking force is obtained.

As is described hereinabove, in the hydraulic pressure control valve of the present invention, the deceleration sensing valve assembly 66 is incorporated with only one of the braking systems, so that trouble in one brake system does not induce malfunctioning of the other brake system. Further by the switching valve assembly 90, sufficient braking force is ensured, even when one brake system fails to operate.

Fig. 4 shows a modification of the switching valve assembly 90 mentioned above. For ease of description, the parts which are substantially the same in construction as those in Fig. 1 are designated by the same numerals. The modified switching valve assembly 90' comprises a regular cylindrical bore 92' formed in the housing 64, and a piston 100' having a land 100'a sealingly and slidably received in the cylindrical bore .92'. As shown, the pressure receiving area defined on the left hand side of the land 1 00'a is the same as that defined on the right hand side thereof. With a spring 114' compressed between the right side of the piston land 100'a and the plug 96, the piston 100' is maintained in its leftmost position, as shown, under the normal operations of the two braking systems and under malfunctioning of one braking system incorporating the inlet and outlet ports 106 and 108. However, when the other braking system malfunctions, the modified switching valve assembly of Fig. 4 functions in a manner identical to the assembly 90 of the preferred embodiment, as described hereinabove.

Claims

1. A hydraulic pressure control valve assembly for a double piping braking system of a wheeled vehicle, comprising: two parallel proportioning valve assemblies which are respectively provided with plungers axially movable in a direction against a common spring in response to the application of hydraulic pressure to respective inlet ports to control the magnitude of hydraulic pressure at respective outlet ports; a deceleration sensing valve assembly which, when the first and second braking systems operate normally, regulates the magnitude of load applied to the common spring in accordance with the degree of deceleration of the vehicle; and a fail-safe means which is so constructed that when the first braking system fails to operate, said means forms a bypass passage between the inlet and outlet ports associated with the second braking system, thereby directly applying the pressure in the second braking system inlet port to the second braking system outlet port, and when said second braking system fails to operate, said means blocks said bypass passage and permits the pressure in the first braking system to compress said common spring, thereby increasing the critical pressure in the first braking system at which the rate of increase of the outlet port pressure of the first braking system to the inlet port pressure thereof changes.

2. A hydraulic pressure control valve assembly as claimed in Claim 1, in which said deceleration sensing valve assembly comprises a piston which supports an end of said common spring, said piston being sealingly disposed in a bore to define a sealed expandable operating chamber which communicates with a ball chamber in which a ball' is movably disposed.

3. A hydraulic pressure control valve assembly as claimed in Claim 2, in which said fail-safe means comprises: a housing formed with a bore and first, second, third and fourth passages which communicate with said bore, said first and second passages being respectively connected to the inlet and outlet ports associated with the second braking system, said third and fourth passages being respectively connected to the inlet port associated with the first braking system and said ball chamber of said deceleration sensing valve assembly;; a piston axially movably disposed in said bore to provide first and second conditions selectively, said first condition being such that said first and second passages communicate but said third and fourth passages are blocked, said second condition being such that said first and second passages are blocked but said third and fourth passages are blocked but said third and fourth passages communicate; and biasing means for biasing said piston to take said second condition.

4. A hydraulic pressure control valve assembly as claimed in Claim 3, in which a pressure receiving area of said piston against which the pressure from the second braking system inlet port applies is smaller than the other pressure receiving area of the piston against which the pressure from the first braking system inlet port applies.

5. A hydraulic pressure control valve assembly substantially as described with reference to, and as illustrated in Fig. 1, or Fig. 4, of the accompanying drawings.