CA1188351A - Vehicle hydraulic brake system and apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A pressure regulating valve for a vehicle fluidic brake system regulates the input to output pressure relation-ship and has a cam whose position selectively restricts the function of the valve, thereby modifying the input to output pressure relationship. The cam includes a rota-table shaft and a coupling between the cam and the shaft for driving the cam in response to the shaft. The coupling includes a lost motion whereby the shaft functions inde-pendently of the cam whenever the cam exhibits a predeter-mined resistive force.

Description

335~

This is a di~ision of Patent Application 375,136, filed April 9, 1981.
The present invention relates to a pressure regu-lating valve for a load sensing hydraulic brake pressure control apparatus for use in the hydraulic circuit between the master cylinder and the rear wheel brake cylinders.
The apparatus is adapted for sensing variations in the distance between the vehicle chassis and the suspended axle shaft.
It is known that changes in vehicle loading cause changes in braking capability. For example, when a vehicle is fully loàded, th~ rear wheels will have nearly the same braking capability as the front wheels. However, when the vehicle is lightly loaded, the rear wheels may exhibit less braking capability than the front wheels. Thus the potential Eor prematu~e rear wheel lock up is much greater when stopping the lightly loaded vehicle than when stopping the fully loaded vehicle. In order to compensate for the inherent imbalance between front cmd rear braking action, .tt has been customary in past years to provide a proportion-lng valve which restricts fluid communication to the rear wheel brake cylinders after a predetermined pressure level is generated. However, such proportioning valves represent a compromise between the desirable system characteristics for the full load condition and those for the light load condition. Thus the selected proportioning valve character-istic is neither suitable for the full load condition nor the light load condition. Many load sensing or vehicle height sensing valve mechanisms have heretofore been pre-sented in the prior art. However, they are unnecessarily complex or otherwise unsuitable for-modern vehicle use.
For example, s2e U.S. Patents: 3,362,758; 3,503,657;
3,649,08~; 3,684,329; 3,734,574; 3,768,1376; 3,848,932;
4,150,855 and 4,159,855.
It is an object of the present invention to pro-vide a novel and improved valve and cam arrangement for use in a vehicle hydL~auIic brake system.
~&

.... . . . . . ...

~8~335~

According to the invention there i.s provided, in a pressure reguIating ~alve for a vehi.cle fluidic brake system wherein the valve includes operable valve means for regulating the input to output pressure relationship and positiona~le cam means communicating with the operable valve means, whereby the position of the cam means in rela-tion to the operable valve means acts to selectively restrict the function of the operable valve means, thereby modifying the input to output pressure relationship, the improvement wherein the positionable cam means includes rotatable shaft means, and cooling means be-tween the cam means and the rotatable shaft means ~or driving the cam means in response to the rotatable shaft means, the coupling means including lost motion means whereb~ the rotatable shaft means functions independently of the cam means whenever the cam means exhibits a prede-termined resistive force.
Preferably, the rotatable cam means includes stop means whereby the angle of rotation o the cam means is limited to a predetermined arc.
In use, the valve may he rigidly attached to a vehicle frame and the cam means is driven by mechanical linkage attached to the vehicle axle. As the vehicle is loaded, compression of the suspension system reduces the distance between the vehicle frame and the axle. The mech-anical linkage, in response to the reduction in distance, rotates the cam means to a position whereby the valve isdisabled. Thus pressure may be passed undisturbed through the valve to the rear wheel brakes.
In a preferred embodiment, the cam means is rota-tingly seated upon the shaft means so as to allow relative xotation thèrebetween and the coupling means comprise atorsional spring a~fi~ed to the cam means and having one leg anchored thereon and the other leg engaging a flat diametric camming surface provided in the shaft means to provide the lost motion. Thus the cam means is caused to rotate in concert wi.th.the shaft means. ~owever, by reason o~ the torsion spring a unique drive mechanism is 3~

provided which accommodates relative motion between the vehicle frame and the axle during vehicle operation by permitting relative rotation between the cam means and the shaft means whenever rotation o~ the cam means is restricted by the functional operation of the valve.
Although the valve is hereinafter described as being in series with another proportioning valve assembly, it is to be understood that the present valve may be used alone in systems where a master cylinder output pressure is suitable~ without an intervening proportional valve, for direct transmission to the vehicle brakes in the heavily loaded condition.
Embodiments of the invention will now be described with reference to the accompanying drawings, in which:
Figure 1 i.s a schematic view of a hydraulic brake system incorporating a load sensing praportionin~ valve embodying the present invention;
Figure 2 is a graphical illustration o~ the per-formance of a brake proportioning system;
~ Flgure 3 pictorially depicts a typical vehicle installation oE a load sensing prc~portion.tng valve embodying the pres0nt invention;
Figure 4 is a partial cross~sectional view of the load sensing proportioning valve used in the brak~ng system illustrated in Figure l;
Figure 5 is a cross-sectional view taken along line 5-5 in Figure 4;
Figure 6 is a partial cross-sectional view taken along line 6-6 in Figure 4;
Figure 7 is a par~ial cross-sectional view taken along line 7~7 in Figure 4;
Figure 8 is an exploded pictorial view showing the assembly of elements comprising the digital cam portion of the load sensing proportioning valve;
3S Figure 9 is an isolated pictorial view of the digital cam rotated 180 from that shown in Figure 8;
Figure 10 is a schematic illustration o~ the load sensing proportioning valve configuration when the 1~83~5~

vehicle is lightly loaded;
Figure 11 is a schematic illustration of the load sensing proportioning valve configuration when the vehicl~
is heavily loaded;
Figures 12 and 13 presen~ schematic illustrations of load sensing proportioning valve configurations accommo-dating over-rotation of the digital cam driveshaft; and Figure 14 presents a partial cross~sectional view of the load sensing proportioning valve, similar to that of Figure 6, wherein the digital cam mechanism is configured activation by clockwise rotation of the digital cam driveshaft.
Referring to the drawings a vehicle hydraulic braking system is shown in Figure 1. Master cylinder 11 provides brake activating hydraulic fluid pressure by means of conduit F to the vehicle fron-t wheel brakes 13L and 13R first passing through a metering valve assembly, not shown, contained in combination valve 12. Condui.t R simi-larly provides an independent source of brake activating hydraulic fluid pressure to a first proportioning valve assembly 14, shown schematically i.n combination valve 12, for supply to the vehicle .rear whee]. brakes 15L and 15R.
Proport.ioning valve 11 may be of any design known to the art, such as shown in U.S. Patent 3,423,936, having a single split point relationship between input hydraulic pressure and output hydraulic pressure. The proportioning valve 14 is designed to produce an output pressure relation-ship to input pressure as shown in Figure 2 and identified as "LOADED". The split point at which valve 14 begins proportioning being indicated as point L. The curve identi-ied as "LOADED" in Figure 2 represents a master cylinder to rear brake pressure relationship acceptable for a vehicle loaded beyond a given mid-load condition and up to its full gross vehicle weight IGVW). The output hydraulic fluid pressure from proportioning valve 14 is transmi-tted to the vehicle rear brakes by conduits Rl and R2 passing through load sensing proportioning valve (LSPV~ device 20.
LSP~ 20 includes a second proportioning valve assembly 16, hereinafter described in greater detail, having 35~
_ 5 a similax construction as that of proportioning valve assembly 14 contained in combination valve 12. Propor-tioning valve assembly 16, when permitted to function, operates upon the output hydraulic pressure received from proportioning valve 14 such -that the relationship between master cylinder pressure (input to proportioning valve 14) to rear brake pressure (output from proportioning valve 16~ is represented by the curve identified as "EMPTY"
in Figure 2. The "EMPTY" curve shown in Figure 2 repre-sents a master cylinder to rear brake pressure relationship acceptahle for a vehicle load condition falling below the selected mid-load condition.
A digital cam mechanism 25 is provided within LSPV 20 to selectively disable proportioning valve assembly 16 in the full open configuration when the vehicle is heavily loaded. Thus when the vehicle is loaded beyond khe selected mid-load condition, proportioning valve 16 is disabled by action of digital cam 25 thereby permitting, undisturbed, the transmission of hydraulic pressure there-through and resulting in the desired "LOADED" pressure relationship shown in E`igure 2. E~owever, when the vehicle is lightly loaded proportioning valves 14 and 16 function ln series and produce a master cylinder pressure to rear brake pressure relationshlp as indicated by the curve "EMPTY" in Figure 2.
Figure 3 pictorially depicts a typical vehicle installation of the load sensing proportioning valve.
LSPV 20 is rigidly afixed to a non-suspended portion of the vehicle frame 35. Driveshaft 50 is firmly attached to linkage 30 so that as linkage 30 rotates driveshaft 50 rotates diyital cam 25 by a drive mechanism hereinafter described in greater detail. Linkage 30 is firmly attached to the vehicle axle tube 31 or any other suitable element of the suspended portion of the rear wheel assembly.
Digital cam 25, through action of linkage 30 attached to vehicle ~xle 31 responds to compression or expansion of the ~ehicle suspension system [not shown).

~8~ 5~

When the lin~age is extended the ~ehicle is lightly loaded and proportioning ~al~e 16 is permitted to function. However, when the linkage is compressed the vehicle is heavily loaded and digital cam 25 is rotated into posi-tion so as to disable the operation of the proportioning valve 16.
Referring to Figure 5, proportioning valve assembly 16 as shown and described herein is merely representative of known proportioning valve mechanisms. Recognizing that any known proportioning valve mechanism which may be modi-fied to Eunction as herein described is suitable for usewith the present invention, the operation of proportioning valve assembly 16 will be described only to the extent necessary to understand its interrelationship with my digital cam and its function with respect to the overall brake hydraulic system.
Proportioning valve assembly 16 comprises valve piston 40 posltioned axially within bore 45 and extending into bore 45a of smaller diameter which in turn opens into digital cam cavity 70. 0-ring seal 47 is provided to hy-draulically seal bore 45 from bore 45a thereby preventingthe 10w of hydraulic fluid into bore 45a. Piston 40 is provided with a pin li]ce extension 4~ projecting into bore ~9. Piston 40 is permitted to axially translate within bore 45a so that pin 48 may project into the digital cam cavity 70 as will be described hereinafter.
The opposite end of piston ~0, includes valve head 43 which is less in diameter than that of bore 45b thus permitting the unrestricted flo~ of hydraulic fluid thereby. Piston 40 is further provided extension cap 41 having notch 42 therein. Piston 40 is normally biased to the laft by action of spring 46 such that extension 41 is urged abuttingly against the end oE bore 45b. Hy-draulic fluid is thus permitted to enter inlet port Rl, freely pass between piston 40 and elastomeric valve sea-t 35 44, past valYe head 43, through notch 42 and exit through outlet port R2. Thus in the configuration as shown in Figure 5 the fluid p~essure at outlet port R2 will be equal to the fluid pressure at inlet port Rl.
During brake application the above-descrihed fluid path through proportioning valve 16 remains open untilthe fluid pressure delivered at inlet port Rl a-ttains a pre~etermined level. ~t this time valve head ~3 will close against valve seat 44. The level of pressure at which this occurs is dependent upon the force of spring 46 as compared to the effective area of the valve piston 40, acted upon by inlet fluid pressure in a direction op-posing the force of spring ~6. This effective area isequal to the diameter D of piston 40 since -the right hand end of piston 40 projecting into bore 45a is sealed off from the inlet fluid pressure by O-ring seal 47 while the inlet fluid pressure acts against all of the remaining portions o piston 40.
After valve head 43 closes against valve seat 44 and the fluid pressure at inlet port ~1 is further in-creased, the increased pressure will act against piston 40 over an effective circular area having a diameter equal to the main sealing diameter of valve head 43 less the cross-sectional area of piston ~0 extending into bore 45a.
Thls produces a force acting upon piston 40 in the same direction as an assisting spring 46 to re-open valve head 43 so as to deliver at least a portion of the increased fluid pressure to outlet port R2. However, any increased fluid pressure delivexed to outlet port R2 creates an op-posing ~orce upon piston 40. The opposing force tends to re-close valve head 43 against valve seat 44. The op-posing forces tend to keep valve head 43 closely adjacent to valv0 seat 44 thereby restricting the flow of fluid from inlet port Rl to outlet port R2 creating a pressure at the outlet port R2 which increases at a lower rate than the pressure at inlet port Rl. The ratio of the pressures is determined by the relationship of the effective areas previously referred to and hence the fluid pressure passing through proportioning valve 16 may ~e proportioned to follow a predetermined relationship.

During that portion of a brake applica-tion in which the applied pedal eEfort is reduced subsequent to a brake application of su~ficient intensity to have moved piston 40 to the restricted flow position the forces tending to move piston 40 to the left are reduced and piston 40 translates to the right under the influence of the pressure at outlet port R2. As the piston 40 moves right valve head 43 is permitted to slide within the inner peripheral surface of valve seat 44, thereby increasing the available volume for the fluid at the rear brake cylinders 15L and 15R and accomplishing a reduction in pressure at outlet port R2. The pressure a-t outlet port R2 can ne~er be greater than the pressure at inlet port Rl because valve seat 44 also acts as a fluid check val~e permitting the flow of fluid from port R2 and into bore 45.
For a more detailed description relating to pro-por-tioning valve operation and the design of particular propor-tioning valve elements refer to U.S. Patent 3,423,936 issued to William Stelzer on ~anuary 28, 1969.
Figures 4 through 9 are to be referred -to ~or the ~ollowing descrlption of the digital cam 25, its con-struction and operation. LSPV houslng 19 is provi.ded with a -two s~t~p bore 60. Floor 69 of bore 60 contains recessed therein semi-circular slot 67 and journal recess 68. Cam ariveshaft 50 is supported and retained as shown in Figure 4. Journal 51 of driveshaft 50 is rotationally received within journal recess 68. Shaft 50 extends generally normal to bore floor 69 passing through and rotationally supported by end cap 61. End cap 61 is snugly retained within bore 30 60a and against shoulder 62 by action of snap ring 63.
O-ring 55 is provided to seal the digital cam chamber 70 from the entrance of any contamination thereto. Cam drive-shaft 50 protrudes externally of end cap 61 sufficiently to permit rigid engagement thereof by linkage 30 (see Figure 3). Thus dri~es~aft 50 is caused to rotate through the same angular displacement as that of linkage 30.
Digital cam 25 i.s rotationally supported on cam 33S~

journal 52 of dri~eshaft 50 such that cam 25 may rotate relative to driveshaft 50. Cam 25 is provided with a peri-pheral recess 26 and axially directed knurls 24 over at least the working peripheral portion of cam 25. The working portion of cam 25 will become apparent as the function and operation are further described hereinafter. Pin 32 projects axially from cam 25 into and slidably engaging slot ~7 in bore floor 69 thereby limiting the angular rota-tion of cam 25 to that are inscribed by slot 67. The in-board side 22 of cam 25 is milled providing inboard fa~ingstepped surface 27. Circular recess 21 extends axially through cam 25 from the outboard surface 28 and slightly past the inboard facing stepped surface 27 thereby pro viding passageway 23 between outboard surface 28 and in-board surface 27. Mandrel 33 is axially positioned withincircular recess 21 extending outboard and slightly pas-t outboard surface 28. Torsion spring 34 is seated about mandrel 33, the helical portion thereof being seated within circular recess 21 such that inboard leg 34a extends through passageway 23 in juxtaposed relation with inboard facing ~tepped surface 27 and engages sprlng retention hole 29.
Outboard spring leg 3~b extends in juxtaposed relation with outboaxd surface 28 of cam 25 e~tending into slot 5~ of driveshaft 50 and enga~ing the flat camming surface 53. In their normal assembled state as hereinabove described and shown in Figure 6, torsion spring legs 34a and 34b are spring loaded so as to apply an angularly outward force upon spring retention hole 29 and the flat camming surface 53 of driveshaft 50. Slo~ 56 is provided at the external and outboard end of cam dri~eshaft 50 to permit external ad~ustment.
In cperation cam 25 is caused to rotate with cam driveshaft 50 by reason of torsion spring 34 applying spring force upon camming surface 53 oE shaft 50. However, should cam 25 be restricted from rotating because of inter-ference between pin 32 and slot 67 or because of interference between cam 25 and pin 48 on ~alve piston 40~ cam driveshaft 335~

50 may however, rotate relative to cam 25 by further com~
pressing torsion spring 34. Thus, a spring drive mechanism is provided between cam driveshaft 50 and digital cam 25 which allows ~or over travel of shaft 50 when rotation of cam 25 is otherwise restricted.
Figures 3, 5 through 7 and 10 depict the con-figuration of LSPV 20 under conditions of light vehicle loading. The vehicle frame 35 is riding relatively high with respect to suspended axle 31. Thus linkage 30 posi-tions digital cam 25 such that peripheral recess 26 permitspin 48 of piston 40 to axially translate in and out of digital cam chamber 70. Proportioning valve 16 is per-mitted to freely function resulting in a master cylinder pressure to rear brake pressure relationship as shown by the curve identified as "EMPTY" in Figure 2.
So long as the vehicle is lightly loaded propor-tioning valve16 is functional. Peripheral slot 26 accommo-dates operation of valve 16. However, should valve piston pin 48 protxude into cam chamber 70 as a result oE vehicle braking and the vehicle encountered an extreme road con-dition causing cam driveshaft 50 to momentarily over rotate from ex~essive compression of the ~ehicle suspension sys-tem, cam 25 will momentarily engage valve piston pin 48 stopping the cam's cou~terclockwise rotation. H~wever, cam dr~veshaft 50 is permitted to continue its counter-clockwise rotation by compression torsion spring 34. Such a condition is illustrated in ~igure 13.
When the vehicle is loaded heavy the suspension s~stem i5 compressed such that the vertical separation between frame 35 and axle 31 is reduced. Linkage 30 assumes a con~iguration as depicted in Figure 11 thereby rotating digital cam 25 counterclockwise as shown. In -this configura-tion the outermost periphery of cam 25 is ro-tated in a position that disables proportioning val~e 16 by pre~enting the free translation piston 40. Thus in the loaded con-dition, as illustrated in Figure 11, the master cylinder pressure to rear brake pressure relationship is as shown . .. . -~ .

3~:~

by the cur~e identified as "LOADED" in Figure 2. So long as the ~ehicle is in the loaded condi-tion the outer peri-phery of cam 25 will remain in the valve piston disabling configuration as illustrated in Figures 11 and 12. In this configuration and when the applied braking load is such that valve piston 40 attempts to translate to the right valve piston pin 48 butts against cam 25 and engages the axial knurls 24 on the outer periphery of cam 25. Thus cam 25 is restrieted from freely rotating. Any further rotation of cam driveshaft 50 resulting from road induced vacillations of axle 31 will be accommodated by compression of torsion spring 34 as illustrated in Figure 12.
The angle A (Figure 6) between the pin 48 center-line and digital eam step 26a determines the vehicle load condition at which proportioning valve 16 is disabled there-fore it is necessary that this angle be accurately fixed.
Angle A is determined for an unloaded vehicle and represents that angle through which driveshaft 50 will rotate as the vehicle is loaded to that mid-loacl condition at which it is desired to change from the "EMPTY" curve to the "LOADED"
eurve as shown in Figure 2. Step 26b is located so as not to interfere with the operation of proportioning valve 16; pin 32 and slot 67 may also be configured so as to limit the clockwise rotation of cam 25 thereby preventing step 26b in~erfering with the operation of proportioning valve 16.
The LSPV as illustrated in Figures 1 through 13 accommodate~ counterclockwise rotation of cam drivesha~t 50 upon eompression of the vehicle suspension system.
However, the LSPV may be easily adapted to accommodate eloekwise xotation as is illustraked in Figure 14. By reloeation of slot 67 as shown in Figure 14 the mechanism is adapted for eloc~wise rotation.
While an embodiment of the in~ention has been --described herein with considerable particularity, it will be unders-tood that the scope of the present invention is to be determined by the appended claims.

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a pressure regulating valve for a vehicle fluidic brake system wherein said valve includes operable valve means for regulating the input to output pressure relationship and positionable cam means communicating with said operable valve means, whereby the position of said cam means in relation to said operable valve means acts to selectively restrict the function of said operable valve means, thereby modifying the input to output pressure rela-tionship, the improvement wherein said positionable cam means includes rotatable shaft means, and coupling means between said cam means and said rotatable shaft means for driving said cam means in response to said rotatable shaft means, said coupling means including lost motion means whereby said rotatable shaft means functions independently of said cam means whenever said cam means exhibits a pre-determined resistive force.

2. In a pressure regulating valve for a vehicle fluidic brake system wherein said valve includes an operable element for regulating the input to output pressure relation-ship and means for selectively restricting the function of said operable element in response to a sensed vehicle operating condition thereby modifying said input to output pressure relationship, the improvement wherein said means for selectively restricting said operable element comprises rotatable shaft means, said shaft means including a D-shaped axially extending portion thereof, circular cam means coaxial with an independently rotatable about said rotatable shaft means, and coupling means including a tor-sional spring having one leg thereof secured to said circu-lar cam means and the other leg thereof drivingly engaging the flat surface of said D-shaped portion of said rotatable shaft means whereby said rotatable shaft means will rotate independently of and relative to said circular cam means whenever the torque of said rotatable shaft means is suf-ficient to cause said torsion spring to yield.

3. A proportioning valve for a vehicle hydraulic brake system having a lock out mechanism whereby said valve may be selectively rendered inoperative for given vehicle load conditions comprising:
valve means including piston means responsive to hydraulic pressure for operating said valve means;
a rotatable shaft having a D-shaped axially ex-tending portion thereof;
rotatable cam means having a cam profile communica-tes with said piston means, said rotatable cam means being coaxial with and rotatable about said rotatable shaft;
coupling means between said rotatable shaft and said rotatable cam means said coupling means including a torsional spring having one leg thereof secured to said rotatable cam means and the other leg thereof drivingly engaging the flat surface of said D-shaped portion of said rotatable shaft whereby said rotatable shaft will rotate independently of and relative to said rotatable cam means whenever the torque of said rotatable shaft is sufficient to cause said torsion spring to yield.

4. A proportioning valve for a vehicle hydraulic brake system having a valve locking mechanism whereby said valve may be rendered inoperative in an open bypass con-figuration under selected vehicle load conditions comprising:
a housing having a first bore and a second bore, the axes of said bores intersecting each other at a right angle, said first bore having therein proportioning valve means including piston means axially extending therethrough and extending into said second bore;
rotatable shaft means coaxial with and extending axially through said second bore, said rotatable shaft means including a D-shaped axially extending portion thereof;

circular cam means coaxial with said second bore axis and extending radially outward from said rotatable shaft means, said circular cam means including a cam profile communicating with said proportioning valve piston means, said cam profile configured to impede movement of said piston means when said cam means is rotated to a given angular position thereby constraining said valve in an open bypass configuration;
coupling means between said rotatable shaft and said circular cam means, said coupling means including a torsional spring having one leg thereof secured to said circular cam means and the other leg thereof drivingly engaging the flat surface of said D-shaped portion of said rotatable shaft means whereby said rotatable shaft means will rotate independently of and relative to said rotatable cam means whenever the torque of said rotatable shaft means is sufficient to cause said torsion spring to yield; and means for rotating said rotatable shaft means in response to vehicle load conditions.

5. A proportioning valve for a vehicle hydraulic brake system having a lock out mechanism whereby said valve may be selectively rendered inoperative for given vehicle load conditions comprising:
valve means including piston means responsive to hydraulic pressure for operating said valve means;
rotatable cam means having a cam profile communi-cating with said piston means, a rotatable shaft, coupling means between said cam and said shaft whereby rotation of said shaft effects rotation of said cam means, said rotatable cam means including stop means whereby the angle of rotation of said cam means is limited to a predetermined arc, said coupling means including resilient means permit-ting said rotatable shaft to rotate through an angle of rotation greater than that of said rotatable cam, said cam profile being configured to impede movement of said piston means when said cam means is rotated to a given angular position thereby locking said valve means in an inoperative configuration, and means for rotating said rotatable shaft in response to vehicle load conditions.

6. A proportioning valve for a vehicle hydraulic brake system having a valve locking mechanism whereby said valve may be rendered inoperative in an open bypass con-figuration under selected vehicle load conditions comprising:
a housing having a first bore and a second bore the axes of said bores intersecting each other at a right angle, said first bore having therein proportioning valve means including piston means axially extending therethrough and extending into said second bore;
rotatable shaft means coaxial with and extending axially through said second bore, circular cam means coaxial with said second bore axis extending radially outward from said rotatable shaft means, said circular cam means including stop means whereby the angle of rotation of said cam means is limited to a predetermined angle, coupling means between said rotatable shaft and said circular cam means whereby rotation of said shaft effects rotation of said circular cam, said coupling means including resilient means thereby permitting said rotatable shaft to rotate through an angle of rotation greater than that of said rotatable cam, said circular cam means including a cam profile communicating with said proportioning valve piston means, said cam profile being configured to impede movement of said piston means when said cam means is rotated to a given angular position thereby constraining said valve in an open bypass configura-tion, and means for rotating said rotatable shaft in response to vehicle load conditions.