BRAKE PRESSURE REDUCING VALVE ASSEMBLY

Abstract: A deceleration conscious brake pressure reducing valve assembly comprises a valve body (2) defining an inlet chamber (3) and an outlet chamber (4) connected by way of passages (9, 10) formed in a piston (7). A ball (15) is mounted within a cage portion (16) of the piston and rests on an inclined ramp surface (17). A valve member (13) is provided for closing the passage (11) by engagement with the seat (12) formed on the piston (7). A spring (9) maintains the components in the illustrated position until the outlet pressure acting over area A0 is sufficient to overcome the force of spring (9). If at this point deceleration of the vehicle has reached a predetermined level, the piston (7) will move to the left and the ball (15) will run up the ramp (17) in order to close the valve (13). If, however, the vehicle is heavily laden so that the pressure at which the piston (7) starts to move is insufficient to produce a deceleration which will move the ball (15) up to the ramp (17), the piston (7), ball (15), and valve (13) will move as a unit to the left and the passage (11) will remain open. The ball (15) will run up the ramp (17) to close the valve (13) when system pressure produces sufficient deceleration. At pressures above cut-in pressure the valve acts as a metering valve to provide a progressive increase in outlet pressure as the inlet pressure rises. The valve is unable to close, regardless of vehicle deceleration or attitude, at pressures below that necessary to move the piston (7) against the force of spring (9). The ball (15) is substantially isolated from hydraulic fluid flow and is accordingly unaffected by changes in fluid viscosity. In the event of very rapid piston (7) movement the inertia of the ball (15) holds the valve member (13) in the illustrated position to ensure rapid closure of the valve.

Description: BRAKE PRESSURE REDUCING VALVE ASSEMBLY This invention relates to a brake pressure reducing valve assembly for use in a vehicle braking system to supply, under certain braking conditions, a brake pressure to the rear brake actuators of the vehicle which is lower than the brake pressure supplied to the front brake actuators. It is well recognized that it is a desirable characteristic for a vehicle braking system for the rear brake pressure to rise at the same rate as the front brake pressure as the brakes are applied until a certain threshold pressure (referred to the cut-in pressure) is reached, and thereafter for the rear brake pressure to rise more slowly than the front brake pressure as the brakes are further applied. This desirable result can be achieved by interposing one or more pressure reducing valves into the brake supply line to the rear brakes. It is known to make such pressure reducing valves purely pressure conscious. A typical known valve of this type is illustrated in Figure 1 , and the output characteristics of this valve are shown by the broken line OAB of Figure 2. Also plotted on Figure 2 are the computed optimum braking characteristics for a vehicle in the driver only and fully laden states. It will be seen that the simple pressure conscious reducing valve illustrated in Figure 1 provides a good match to the ideal characteristics in the driver only case, but a poor match to the ideal characteristics in the fully laden case. The result of this is substantial under utilization of the available rear brake effort in the fully laden case. It is also known to produce pressure reducing valves which are responsive to deceleration of the vehicle to which the valve is secured. A typical known valve of this type is illustrated in Figure 3 and will be seen to comprise a valve body in which is housed a stepped piston and a ball. The valve body is orientated on the vehicle such that the ball rests on a slope which extends upwardly in the forward direction of the vehicle. In use, inlet pressure is freely communicated to the outlet of the valve until the deceleration of the vehicle reaches a level at which the ball rolls up the slope to close a central passage provided in the stepped piston. Any further increase in inlet pressure produces no increase in outlet pressure until the inlet pressure acting over the smaller diameter of the stepped piston is able to move the piston against the outlet pressure acting over the larger area of the piston. The characteristics of such a valve are illustrated in Figure 4. Under driver only conditions the outlet pressure rises at the same rate as the inlet pressure until point C is reached whereupon the brake pressure is sufficient to produce the deceleration required to move the ball to isolate the outlet from the inlet. A further increase in inlet pressure produces no increase in outlet pressure until point D is reached, whereupon a further increase in inlet pressure is accompanied by an increase in outlet pressure at a rate lower than the increase in inlet pressure. Under fully laden conditions when the brakes are applied the inlet pressure rises at the same rate as the outlet pressure until point F is reached at which point the brake pressure is able to produce sufficient deceleration to move the ball to isolate the inlet from the outlet. Thereafter the outlet pressure is maintained constant whilst the inlet pressure increases until point E is reached, whereupon the outlet pressure again increases but at a rate more slowly than the increase in inlet pressure. It will be seen that the characteristics plotted on Figure 4 provide a slightly better match to the ideal characteristics in the fully laden case than the characteristics of Figure 2. However, deceleration conscious valves of the type illustrated in Figure 3 suffer from a number of well recognized disadvantages. Firstly, although the characteristics of the valve are closer to the ideal fully laden characteristics than those produced by a purely pressure conscious reducing valve, the match is still not good and accordingly there is under utilization of the potential rear brake effort available under fully laden conditions. This poor match is at least in part due to the fact that after the ball has moved to isolate the inlet from the outlet a substantial further increase in inlet pressure is necessary before there is any increase in outlet pressure. Also, under certain driving conditions, in particular when negotiating steep downward hills it is possible for the ball to roll into engagement with the end of the piston either at relatively low braking pressures, or even before application of the brakes. If, for example, a driver is descending a steep hill, engages a low gear and thereby produces a transient deceleration of the vehicle and applies the brakes during that transient deceleration, it is possible that the ball will already be in engagement with the piston when the brakes are applied. This will cause the outlet pressure to rise along the straight line ODE producing very substantially under utilization of the rear brakes. It will be appreciated that such under utilization is experienced at precisely the time when optimum braking efficiency is required - i.e. whilst descending a steep hill. Various practical designs of deceleration conscious reducing valve suffer from the additional disadvantage that in order to prevent excessive rear brake pressure during extremely rapid application of the brakes the valve is designed to make the ball move in part in response to the rate of fluid flow between the inlet and the outlet. This is difficult to achieve in practice, and consistent results are especially difficult to achieve because of the varying viscosity of the brake fluid at varying temperatures. The preferred embodiments of the present invention overcome or obviate the problems outlined above and provide a reasonably simple and inexpensive valve which offers substantially improved characteristics as compared with both pressure conscious reducing valves and purely deceleration conscious reducing valves. According to one aspect of the present invention a brake pressure reducing valve assembly for a vehicle braking system comprises: a valve body; an inlet chamber defined within the valve body and connectable to a source of brake actuating fluid; an outlet chamber defined within the valve body and connectable to a brake actuator; normally open valve means establishing communication between the inlet and outlet chambers; a deceleration responsive member for closing the valve means in response to deceleration of the vehicle to which the valve assembly is attached exceeding a threshold value; and means for preventing closure of the valve by the deceleration responsive means if the pressure in the outlet chamber is below a predetermined value. The means for preventing closure of the valve means eliminates the problem of premature closing of the valve means under downhill driving conditions, as outlined above. In the embodiments of the present invention the valve will remain open to provide rear brake pressure equal to front brake pressure until a certain threshold pressure is reached, regardless of the operation of the deceleration responsive member. In the preferred embodiment of the invention, the predetermined pressure corresponds to the desired cut-in pressure in the driver only case. Preferably, the valve means is a metering valve which is operative in response to an increase in inlet pressure occurring after closure thereof to provide an increase in outlet pressure which is smaller than the increase in inlet pressure. Accordingly, in the preferred embodiment of the invention after the valve means has been closed by the deceleration responsive member a further increase in inlet pressure will result in an increase in outlet pressure rather than the static outlet pressure characteristic of deceleration conscious reducing valves of the type illustrated in Figure 3. In a particularly preferred embodiment of the invention the threshold level of deceleration necessary to make the deceleration responsive member close the valve means is not constant under all conditions, but rather increases as the outlet pressure at the moment when the threshold value is attained increases. In other words, if a brake pressure higher than the predetermined value is necessary to produce a deceleration sufficient to cause the deceleration responsive member to close the valve means, then the deceleration required in order to make the deceleration responsive member close the valve means is higher than the value which would have been necessary had the deceleration produced by the predetermined value of outlet pressure been sufficient to cause the deceleration responsive member to close the valve means. This characteristic will enable the valve to have an output characteristic much better matched to the ideal characteristic in the fully laden case. In effect, the cut in pressure will be determined not only by the deceleration of the vehicle, but also by the brake pressure necessary to obtain that deceleration. If a high level of brake pressure is required to produce a particular deceleration, indicating that the vehicle is fully laden, the cut in pressure is increased in order to provide for better utilization of the large rear braking effort available for a fully laden vehicle. In particularly preferred embodiments of the invention the deceleration responsive member is located such that there is no flow of fluid past the member during braking, and accordingly movement of the member will not be significantly affected by the rate of flow of brake fluid or the viscosity of the brake fluid. In a particularly preferred embodiment of the invention the deceleration responsive member engages the valve member of the valve means in the manner such that it is necessary to overcome the inertia of the deceleration responsive member in order to move the valve member. This arrangement offers the particular advantage that if the cooperating valve seat is moved extremely rapidly towards the valve member as a result of an extremely rapid rise in brake pressure, the inertia of the deceleration responsive member will assist in holding the valve member in place to cooperate with the rapidly moving valve seat. By this means, any tendency for the valve member to be moved along with the rapidly moving valve seat rather than closing against the valve seat will be substantially reduced, with the result that the tendency in present deceleration conscious valves for an excessively high pressure to be trapped downstream of the valve if the brakes are applied exceedingly rapidly is avoided. The invention will be better understood from the following description of preferred embodiments thereof, given by way of example only, reference being had to the accompanying drawings wherein: Figure 1 illustrates schematically a prior art pressure conscious pressure reducing valve; Figure 2 is a graph of outlet pressure plotted against inlet pressure illustrating ideal valve characteristics for the fully laden and driver only case, together with the actual characteristics produced by the valve of Figure 1 ; Figure 3 illustrates a prior art deceleration conscious pressure reducing valve; Figure 4 is a graph corresponding to Figure 2 illustrating the characteristics of the valve of Figure 3; Figure 5 illustrates a first embodiment of the present invention; Figure 6 is a graph corresponding to that of Fig. 1 illustrating the characteristics of the valve of Fig. 5; Figure 7 shows a second embodiment of the present invention; Figure 8 is a graph corresponding to that of Fig. 6 illustrating the characteristics of the valve of Fig. 7. Figure 9 shows a third embodiment of the invention; and Figure 10 is a graph showing the characteristics of the value of Fig. 9. Referring firstly to Figure 5 the brake pressure reducing valve 1 illustrated comprises a valve body 2 in which is defined an inlet chamber 3 and an outlet chamber 4. The inlet chamber 3 is connectable by way of an inlet fitting 5 to a source of brake actuating fluid, for example a hydraulic master cylinder. The outlet chamber 4 is connected by way of an outlet fitting 6 to a brake actuator, for example a drum brake slave cylinder. A piston 7 is slidably mounted in the valve body and has a relatively large area A2 exposed to the outlet chamber 4 and a relatively small area AQ exposed to atmosphere via a vent passage 8 formed in the valve body. An annular zone A-j representing the difference between A2 and AQ is exposed to the working pressure in the inlet chamber 3. The piston 7 is biased towards the outlet chamber 4 by a spring 9 and provides via a radial bore 10 and an axial bore 11 a passageway connecting the inlet chamber 3 to the outlet chamber 4. One end of the passage 11 forms a valve seat 12 with which a valve member 13 can cooperate to close the passage 10,11 to fluid flow. In the rest position of the valve as illustrated in Figure 5 the valve member 13 abuts the end wall 14 of the valve body and is abutted by a ball 15 retained within a cage-like central portion 16 of the piston. The piston is biased into contact with the ball 15 by the spring 9 with the result that, in the illustrated rest position, the valve member 13 is held clear of the valve seat 12 and free communication exists between the inlet chamber 3 and the outlet chamber 4. The valve assembly 1 is mounted on the vehicle body as illustrated with the longitudinal axis of the piston sloping upwardly towards the front of the vehicle. In use, if the brakes are applied at moderate rate fluid will flow from the inlet fitting 5 through the inlet chamber 3, radial passage 10, bore 11 , and outlet chamber 4 to the outlet fitting 6 and during an initial phase of pressure increase the pressure at the outlet 6 will be substantially the same as the pressure at the inlet 5. The increase in brake pressure will retard the vehicle. When the brake pressure acting over area Ag is able to overcome the force of spring 9 the piston 7 will move away from the outlet chamber 4. If the vehicle is loaded only with the driver the brake pressure necessary to move the piston 7 as described above will be sufficient to produce a threshold deceleration which will cause the ball 15 to run up a ramp surface 17 provided by the cage 16 as the piston 7 moves away from the outlet chamber 4. Accordingly, as the piston moves away from the outlet chamber 4 the valve member 13 will be held in the illustrated position by the ball 15 and will engage the seat 12 to interrupt communication between the inlet chamber 3 and the outlet chamber 4. If there is a further increase in brake inlet pressure this will be applied to the piston 7 over the area A-| and the piston 7 and valve member 13 will accordingly act as a metering valve to produce an increase in pressure at the outlet 6 which is less than the increase in pressure at the inlet 5. The characteristics so far described is represented by the line OXY of Figure 6. If, as a result of the vehicle being loaded by more than the driver only the deceleration of the vehicle produced by the pressure sufficient to cause the piston 7 to move away from the outlet chamber as described above is insufficient to roll the ball 15 up the ramp 17, the ball 15 will remain in its illustrated position relative to the piston 7 as the piston moves away from the outlet chamber. Accordingly, the valve member 13 will tend to move with the piston 7 and will move out of contact with the end wall 14 of the valve body. Accordingly, the communication between the inlet chamber 3 and outlet chamber 4 will not be interrupted when the piston 7 begins to move. If the brake pressure is further increased the deceleration of the vehicle will increase until a point at which the ball 15 tends to roll up the ramp 17 thereby moving the valve member 13 to engagement with the seat 12 and interrupting communication between the inlet chamber 3 and outlet chamber 4. Thereafter, if there is any further increase in inlet pressure this will act over the area A-| to drive the piston 7 back towards the outlet chamber 4 and increase the pressure at the outlet 6 by an amount less than the increase at the inlet 5. Eventually, if the movement of the piston 7 towards the outlet chamber 4 is sufficient to bring the valve member 13 into contact with the end wall 14, the valve member 13 will be lifted from the seat 12 and will act as a metering valve to continue the supply of pressurized fluid to the outlet 6. Under fully laden conditions, the characteristics of the valve of Figure 5 are as represented by the line OXZY of Figure 6. It will be noted that it is impossible for the valve 13 to engage the seat 12 until the pressure within the valve body is sufficient, acting over the area AQ # to overcome the force of spring 9. Accordingly, the problem of the prior art outlined above in which the deceleration conscious member was able to isolate the inlet from the outlet before any application of brake pressure is totally avoided. It will also be noted that in order to move the valve member 13 the ball 15 must also be moved. Accordingly, before the member 13 can be moved the inertia of the ball 15 must be overcome. If the brakes are applied extremely rapidly it is possible for the piston 7 to move extremely rapidly in the direction away from the outlet chamber 4. If this happens, the ball 15 will tend to maintain the valve member 13 in the illustrated position and accordingly the valve seat 12 will very rapidly engage the valve member 13 to isolate the outlet chamber 4 from the inlet chamber 3. This is particularly desirable since it prevents an excessively high pressure being trapped downstream of the valve as was possible in the case of prior art deceleration conscious reducing valves. It will further be noted that although the ball 15 is mounted within the inlet chamber 3 it is substantially isolated from the dynamic effects of fluid flowing through the inlet chamber 3 and accordingly operation of the valve will not depend on fluid flow rates or on the viscosity of the hydraulic fluid. Referring now to Figure 7 there is illustrated a second embodiment of the invention. In this case the valve body 2A defines an inlet chamber 3A and an outlet chamber 4A. The inlet chamber is, in use, connected to a source of braking actuating fluid via an inlet fitting 5A and the outlet chamber 4A is connected to a brake actuator by an outlet fitting 6A. A stepped piston 7A is slidably mounted in the valve body and is biased towards the outlet chamber by a spring 9A. A series of radial and axial passages 10A, 11A, provide a normally open communication between the inlet chamber 3A and the outlet chamber 4A. A valve member 13A can cooperate with a valve seat 12A formed on the piston to interrupt the communication between the inlet chamber 3A and outlet chamber 4A. The valve member 13A is coupled by way of a ring 18 to a ball 15A which is mounted within the outlet chamber 4A and rests on a ramp surface 17A. The ball 15A is a lose fit within the ring 18 so that the ball 15A is able to roll on the ramp surface 17A in use of the valve as described below. In use, the valve is mounted as illustrated in Figure 7 with the axis of the piston sloping upwardly towards the front of the vehicle. In the rest position the components are biased into the illustrated positions by the spring 9A and there is free communication between the inlet 5A and the outlet 6A via the inlet chamber 3A, passages 10A,11A, and outlet chamber 4A. As inlet pressure rises the outlet pressure rises at the same rate until the pressure within the valve body, acting over the area A Q is sufficient to overcome the force of the spring 9A, whereupon the piston will move away from the outlet chamber 4A. If, at this pressure, the deceleration of the vehicle is sufficient to maintain the ball 15A at the upper end of the ramp surface 17A, the ball will act via the ring 18 on the valve member 13A to hold the valve member 13A in the illustrated position. Accordingly, movement of the piston 7A will bring the seat 12A into engagement with the valve member 13A and communication between the inlet chamber and the outlet chamber will be interrupted. Thereafter, any further increase in inlet pressure will cause the valve member 13A to act as a metering valve and produce the desired increase in outlet pressure at a rate less than the increase in inlet pressure. If, due to the loading of the vehicle, the deceleration produced by the brake pressure sufficient to move the piston 7A against the force of the spring 9A is insufficient to maintain the ball 15A at the upper end of the ramp surface 17A, the ball 15A will roll down the ramp surface and follow the piston 7A as it moves. Accordingly, the valve member 13A will be held out of engagement with the seat 12A, and further increase in inlet pressure will be communicated in full to the outlet 6A. When the outlet pressure rises to a level at which the vehicle is decelerating sufficient to move the ball 15A up the ramp 17A, the ball will move to bring the valve member 13A into contact with the seat 12A, and the valve will thereafter operate as a metering valve as described above. The various advantageous characteristics of the valve of Figure 5 are also obtained from the valve illustrated in Figure 7. A further particular advantage of the valve of Figure 7 arises if the ramp surface 17A moves away from the axis of the piston in the direction towards the rear of the vehicle - i.e. if the gradient of the ramp surface 17A increases towards the lower end of the ramp surface. The advantage of this arrangement is that if the vehicle is laden above the driver only load so that the deceleration of the vehicle is insufficient to maintain the ball 15A at the upper end of the ramp 17A when the piston 7A begins to move, the deceleration necessary to move the ball 15A back up the slope 17A will be progressively higher the further the ball rolls down the ramp surface 17A. The effect of this is to make the cut-in pressure of the valve dependent not only on deceleration of the vehicle, but also on the level of brake pressure necessary to achieve a particular level of deceleration. If the vehicle is fully laden, for example, the piston 7A will have moved a substantial distance away from the outlet chamber in order to produce the level of deceleration which, in the driver only case, would have been sufficient to maintain the ball 15A at the upper end of the ramp surface 17A. However, because by this time the ball 15A will be on a steeper portion of the ramp surface 17A, that level of deceleration will be insufficient to move the ball 15A back up the ramp surface, and accordingly an even higher level of brake pressure will be necessary in order to move the ball 15A. The effect of this arrangement is to shift the fully laden cut-in pressure substantially nearer the fully laden ideal characteristic, thereby substantially improving the utilization of the rear brakes under fully laden conditions. The characteristics of the valve of Figure 7 are shown in Figure 8, on which the same reference characters as those used in Figure 6 have been used. It will be noted that the driver only characteristics of the Figure 7 valve correspond to those of the Figure 5 valve. However, the point Z of the Figure 7 valve, i.e. the fully laden cut-in pressure, is closer to the ideal fully laden characteristic than is the corresponding point of the Figure 5 valve, thereby producing a better match to the fully laden ideal characteristic. Whilst the invention has been particularly described with reference to the use of a ball as the deceleration responsive member it will be appreciated that any other suitable deceleration responsive member, for example a roller, may be used instead of a ball. Referring now to Figure 9 a further embodiment of the invention is illustrated. In this embodiment the reducing valve 100 comprises a valve body 102 in which is defined an inlet chamber 103 and an outlet chamber 104. The inlet chamber 103 is connectable by way of an inlet fitting 105 to a source of brake actuating fluid, for example a hydraulic master cylinder, and the outlet chamber 104 is connected by way of an outlet fitting 106 to a brake actuator. A piston 107 is slidably mounted in the valve body and has a relatively large area exposed to the outlet chamber 104 and a relatively small area A Q exposed to atmosphere via a vent passage 108 formed in the valve body. An annular zone A-j representing the difference between A2 and AQ is exposed to the working pressure in the inlet chamber 103. The piston 107 is biased towards the outlet chamber 104 by a spring 109 and includes an axially extending passage 111 which connects the inlet chamber 103 to the outlet chamber 104. One end of the passage 111 forms a valve seat 112 which can be engaged by the surface 113 of a ball 115 which is located within a cage-like central portion 116 of the piston 107. In the normal, illustrated, configuration of the components the piston 107 is biased into contact with a shoulder 110 of the valve body 102 by the spring 109, and the ball 115 is held away from the seat 112 by gravity and by a spring loaded plunger 114. In use, the passage 111 provides a free communication between the inlet 105 and the outlet 106 until the pressure within the valve body rises to a level which, acting over the area A Q , is able to overcome the force of the spring 109 and move the piston 107 to the right. As in the embodiments described above, in the driver only case the brake pressure necessary to move the piston 107 against the force of the spring 109 will produce a deceleration sufficient to bias the ball 115 to the left as the piston 107 moves towards the right. Accordingly, in the driver only case the ball will remain in contact with the plunger 114 as the piston 107 moves to the right, and after the piston is moved by the normal clearance distance between the surface 113 and the seat 112, the seat 112 will engage the surface 113 and communication from the inlet 105 to the outlet 106 will be interrupted. If there is a further increase in pressure at the inlet 105, this increased pressure, acting over the area A-j will tend to move the piston 107 back towards the left. The strength of the spring 118 which acts on the plunger 114 is chosen such that the higher pressure in the inlet chamber 103 acting on the ball 115 over the area of the seat 112 is sufficient to compress the spring 118 as the piston 107 again moves to the left. In other words, once the seat 112 has engaged the ball 115, the ball and piston will move to the left as a unit if there is any further increase in inlet pressure, and the spring 118 will be insufficient to push the ball clear of the seat. ' Accordingly, once the piston is moved sufficiently far to the right for the seat to engage the ball in the driver only case (point X on Figure 10), any further increase in inlet pressure will move the piston and ball together to the left to increase the outlet pressure at a rate lower than the rate of increase in inlet pressure. This will continue until point Y on Figure 10, at which point the piston will again have engaged the shoulder 110. Any further increase in inlet pressure will thereafter merely push the ball 115 more firmly into engagement with the seat 112, and there will be no corresponding rise in outlet pressure. Accordingly, the driver only characteristic OXYZ of Figure 10 is produced. As in the above described embodiments, in the fully laden case the deceleration produced by the pressure necessary to move the piston 107 against the force of the spring 109 will produce insufficient force on the ball to hold the ball 115 stationary relative to the valve body 102 as the piston moves, and movement of the piston 109 to the right will be accompanied by corresponding rightward movement of the ball 115. Only when the inlet pressure has risen to a level sufficient to roll the ball 115 up the slope 117 or 117 (depending on the exact loading of the vehicle) will the ball 115 be able to engage the seat 112 to interrupt communication between the inlet 105 and the outlet 106. Accordingly, in the fully laden case the cut-in pressure of the valve will be increased as compared with the driver only case. In the fully laden case, any increase in pressure at the inlet over and above the cut-in pressure will result in leftward movement of the piston 107 and the ball 115 as a unit to displace fluid to the rear brakes. The rise in pressure at the outlet will be less than the rise in pressure at the inlet as will be understood by those skilled in the art. Because of the relatively large amount of leftward displacement available under these conditions, it is most unlikely that the piston 107 will engage the shoulder 110, and accordingly there will be no upper limit on the pressure supplied to the rear brakes. Accordingly, the valve will produce the characteristic OXAB of Figure 10 in the fully laden case. In the embodiment of Figure 9 a plunger 119 is slidably mounted within a bore provided in the piston 107 and carries a cross pin 120 which is located in an over¬ sized hole 121 in the piston. The cross pin 120 projects beyond the piston so that after the piston 107 has moved by a predetermined amount the cross pin 120 strikes an end face 22 of the valve body. Thereafter, further rightward movement of the ball 115 is prevented, regardless of further rightward movement of the piston 107, and regardless of the deceleration of the vehicle. The effect of this arrangement is to provide a limit to the cut-in pressure, regardless of vehicle deceleration. Once the brake pressure has reached a value sufficient to move the piston 107 by the amount of the initial clearance between the pin 120 and the end face 122 plus the amount of initial clearance between the ball 115 and the seat 112, the seat 112 will engage the ball 115, regardless of deceleration. This arrangement is particularly useful since it provides an upper limit of cut-in pressure to the rear brakes, regardless of the efficiency of the front brakes. Thus, if in the fully laden case the front brakes are producing significantly less retardation than normal, for example as a result of brake pad heating due to repeated use, a cut-in pressure will be reached even though the deceleration produced at the cut-in pressure is insufficient to move the ball 115 up the ramp 117,117A. Accordingly, premature locking of the rear brakes is avoided.

Claims: CLAIMS 1. A brake pressure reducing valve assembly for a vehicle braking system comprising: a valve body; an inlet chamber defined within the valve body and connectable to a source of brake actuating fluid; an outlet chamber defined within the valve body and connectable to a brake actuator; a piston slidably mounted in the body separating the inlet chamber from the outlet chamber, the piston being urged in one axial direction by fluid pressure within the valve body upon initial application of the brakes and in the opposite axial direction by a bias spring; a valve seat provided on the piston; a valve member cooperating with the valve seat to form a normally open valve means establishing communication between the inlet and outlet chambers; and a deceleration responsive member for closing the valve means in response to deceleration of the valve assembly exceeding a threshold value, the arrangement being such that in the absence of fluid pressure within the valve the spring maintains the piston at a position in which the valve seat cannot be engaged by the valve member and that when fluid pressure within the valve body rises to a predetermined value the piston is moved by the fluid pressure against the bias of the spring to a position in which the valve seat can be engaged by the valve member. 2. A brake pressure reducing valve assembly according to claim 1 wherein the position of the valve member relative to the valve seat is controlled, when the pressure in the outlet chamber is above the said predetermined value, by the deceleration responsive member. 3. A brake pressure reducing valve assembly according to claim 1 or claim 2 wherein the inertia of the deceleration responsive member resists movement of the valve member whereby the valve member is engaged by the seat if the piston moves very rapidly regardless of the deceleration of the vehicle. 4. A brake pressure reducing valve assembly according to claim 2 wherein the deceleration responsive member is operative to maintain the position of the valve member fixed relative to the valve body as the piston moves at the predetermined pressure value if the deceleration of the vehicle produced by the said predetermined pressure value is at least equal to a threshold level, whereby the valve member is able to engage the valve seat. 5. A brake pressure reducing valve assembly according to claim 4 wherein the deceleration responsive member permits the valve member to move with the piston as the piston moves at the predetermined pressure value if the deceleration of the vehicle produced by the said predetermined pressure value is less than the said threshold level, whereby the valve member remains spaced from the valve seat. 6. A brake pressure reducing valve assembly according to claim 5 wherein the deceleration responsive member is located within a cage-like portion of the piston and rests on a sloping surface of the valve body. 7. A brake pressure reducing valve assembly according to any preceding claim wherein the deceleration responsive member rests on a sloping surface which diverges from the axis of movement of the piston towards the rear of the vehicle so that if, in use, the deceleration responsive member rolls towards the rear of the vehicle it rests on a sloping surface which is steeper than the sloping surface on which it rests when there is no fluid pressure within the valve body. 8. A brake pressure reducing valve assembly according to any of claims 2-7 wherein the valve member is constituted by a portion of the deceleration responsive member. 9. A brake pressure reducing valve assembly according to any preceding claim wherein after the normally open valve means has closed the reducing valve assembly is responsive to an increase in pressure at the inlet chamber to produce a smaller increase in pressure at the outlet chamber. 10. A brake pressure reducing valve assembly according to any preceding claim wherein the deceleration responsive member is located out of the flow path of fluid flowing through the reducing valve assembly.

Inventors: FARR GLYN PHILLIP REGINALD, FARR, GLYN, PHILLIP, REGINALD
Applicants: LUCAS IND PLC, LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: B60T8
IPC codes: B60T8