WO1983001929A1

WO1983001929A1 - Anti-lock brake modulator, apparatus and method

Info

Publication number: WO1983001929A1
Application number: PCT/US1982/001663
Authority: WO
Inventors: Inc. Garlock; H. Lee Mccord
Original assignee: Garlock Inc
Priority date: 1981-12-04
Filing date: 1982-11-24
Publication date: 1983-06-09
Also published as: EP0095489A1

Abstract

An anti-lock brake apparatus, method and article including a compact, fast, modulator (10) in a brake fluide conduit between a master cylinder reservoir and a brake cylinder. The modulator includes a pump (80, 88) for pumping fluid back from the brake cylinder to the reservoir in response to a particular sensed rate of retardation and a valve (112) for isolating the pump from the brake fluide passageways through the modulator during non-pumping periods. The modulator includes a plurality of separate passageways therethrough, one for each wheel or set of wheels and releases pressure in one wheel while reapplying it to another while the pump is continuously running, without one action affecting the other.

Description

ANTI-LOCK BRAKE MODULATOR, APPARATUS AND METHOD

TECHNICAL FIELD This invention relates to anti-lock brake systems for rotating members, such as the wheels of an automotive vehicle, and in one embodiment to an improvement in the modulator of such systems.

BACKGROUND Anti-lock brake systems are well-known, see for example those described in U.S. Patents 3,989,125,

3,833,097, A, 138, 165 and 4,152,030, incorporated herein by reference. The system described in U.S. Patent 4,068,904 employs a modulator interposed in a conduit between a reservoir and a brake cylinder means for normally accomodating free flow of fluid therebetween and also having means, responsive to a sensor detecting a certain rate of retardation of a wheel, for pumping fluid from the brake cylinder back toward the reservoir for releasing the braking force. U. S. Patent 4,138,165 describes a pumping anti-lock device including means for controlling the rate of rise of brake fluid pressure.

It is an object of the present invention to provide an improved anti-lock brake system.

It is a further object to provide such a brake system having a modulator with the three separate states of: (1) pressure reduction, (2) pressure hold, and (3) pressure reapplication.

It is another object to provide a modulator with a solenoid controlled isolation valve to isolate the pumping means from the brake fluid passageway through the modulator. It is another object of the invention to isolate the volumetric expansion and contraction motion of a piston from the main check valves.

It is a further object to allow the pumping motor to run continuously after the first cycle and to greatly increase the efficiency and perforcance of an anti-lock system, and in a multi-channel configuration (see Figs. 3. 4A and 43 of U.S. Patent 4,068,904) to allow each channel to be controlled completely independently of the others. It is another object of the invention to provide a compact, fast, efficient modulator having a single motor operating a plurality of pistons.

It is a further object to provide a modulator with reapplication control means and pressure hold means that are operative without having to shut off the modulator motor.

It is a further object to provide a modulator with a single motor that allows the pressure to be reduced at one wheel while reapplying or holding pressure to another wheel, without the one act disturbing the other. BRIEF SUMMARY OF THE INVENTION

In a modulator of the type described in U.S. Patent 4,068,9.04 having means for pumping brake fluid back from a wheel brake cylinder to a master cylinder reservoir upon a particular rate of retardation of a wheel being sensed, the improvement including means for controllably isolating a pumping chamber from the brake fluid passageway through the modulator. In a preferred embodiment, the isolating means is a solenoid controlled valve which is opened during pumping periods and closed during non-pumping periods.

In addition, a modulator havins three seυarate states during an anti-lock cycle, namely: (1) a pressure reduction state, (2) a pressure hold state, and (3) a pressure reapplication state. It is noted that the pump motor of the modulator can run continuously not only during the pumping state (1), but also during states (2) and (3), that is. during pressure holding and pressure reapplication.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully under stood by reference to the following detailed description thereof, when read in conjunction with the attached drawings, wherein like reference numerals refer to like elements and wherein;

Fig. 1 is a diagrammatic view of the modulator of this invention as installed in an automobile;

Figs. 2, 3 and 4 are. front, top and rear views, respectively, of a modulator according to a preferred embodiment of this invention;

Fig. 5 is a partly cross-sectional, partly diagrammatic view of the modulator of Figs. 2-4 taken along line 5-5 of Fig. 3;

Fig. 6 is a partly cross-sectional, partly diagrammatic view of the modulator of Figs. 2-4 taken along line 6-6 of Fig. 3; Fig. 7 is a partly cross-sectional, partly diagrammatic view of the modulator of Figs. 2-4 taken along line 7-7 of Fig. 2;

Fig. 8 is a partly cross-sectional, partly diagrammatic view of the modulator of Figs. 2-4 taken along line 8-8 of Fig. 2;

Fig. 9 is an enlarged cross-sectional view showing vthe details of a portion of Fig. 7 and taken along line 9-9 of Fig. 2;

Fig. 10 is an enlarged cross-sectional view showing the details of a portion of Fig. 8;

Fig. 11 is a partly cross-sectional partly diagrammatic view of a portion of Fig. 2 taken along line 11-11 thereof; and

Fig. 12 is an enlarged, cross-sectional view identical to Fig. 9 except that certain parts are shown after movement to a different nosition. DETAILED DESCRIPTION OF THE INVENTION

With reference now to the drawings, Fig. 1 shows a modulator 10 in accordance with the present invention for use in an automobile. The automobile has four wheels 12, 13, 14 and 15. each having a brake 16, 17, 18 and 19 respectively. The automobile also has a logic system 20 for detecting the rate of retardation of rotation of each of the wheels and for signalling impending wheel lock. The logic system 20 includes a sensor 22, 23. 24 and 25 .at each of the wheels 12, 13, 14 and 15, respectively, a control module 26 connected to the modulator 10 and to an automobile battery 28, and a warning indicator 30 which "lights" when the logic system 20 signals the occurrence of a system malfunction.

Any known sensor system can be used, such as one using a magnetic pickup or sensor at each wheel which feeds velocity information to a microcomputer which makes decisions about the impending lock and when the rate of retardation of rotation of a particular wheel reaches a certain value, it generates a signal which then causes the modulator to be energized, for example, to reduce fluid pressure in that wheel. Such logic systems are well-known in the art and therefore are not described here. The modulator 10 is connected to a master brake cylinder reservoir 32 by two hydraulic fluid conduits 34 and 36 and is connected to the four brakes 16-19 by four separate conduits 38, 39, 40 and 41 respectively.

The modulator 10 will .now be described in detail with reference to Figs. 2-12. Referring first to Figs. 2-4 showing the outside of the modulator, Fig. 2 is a front view, Fig. 3 is a top view, and Fig. 4 is a rear view of the modulator 10. The modulator 10 includes a modulator body 50 (Fig. 2) having a front plate 52 attached thereto by screws 54 and a rear plate 56 attached thereto by screws 58. The modulator also includes four through-bolts 60, see Figs. 4, 5, 6 and 11. Referring now to Figs. 5-12, the modulator 10 includes four equally spaced-apart identical, solenoid controlled re-application valves 62. 64, 66 and 68 in the front of the modulator (Figs. 5 and 7), and four equally spaced-apart, identical, solenoid controlled pressure reduction control valves 70, 72, 74 and 76 in the rear of the modulator (Figs. 6, 7 and 8). The modulator also includes four (only three are shown) identical pistons 78, 80 and 82 (Figs. 8 and 10) mounted for reciprocating movement and driven by a motor 86 and a cam 88 (Fig. 8).

The front plate 52 includes a pair of inlet ports 90 and 92 (Figs. 2, 3, 7 and 9) and the rear plate 56 includes four outlet ports 94, 96, 98 and 100 (Figs. 4. 7 and 8). There are four substantially identical, independent, fluid paths through the modulator 10, one for each of the wheels 12-15. Each path goes through one reapplication valve and one pressure reduction valve and is associated with one piston..

A description of one path will suffice because all four are substantially identical. For purposes of this description, the path going through the reapplication valve 68, the pressure reduction valve 74 and associated with the piston 82 will be described in detail. Referring mainly to Fig. 7, this path includes (and starts with) fluid inlet port 92, a passageway 102 from port 92 to the reapplication valve 68, from reapplication 68 through passageway 104 to the pressure reduction valve 74 and from, pressure reduction valve 74 to the outlet port 96. Associated with this path is an expansible-contractible chamber 106 (see Fig. 10) which includes a passageway 108 and a cylinder 110 in which the piston 82 reciprocates. Fluid communication between the chamber 106 and the pressure reduction valve 74 is controlled by an isolation valve 112. Regarding path 104 shown in Fig. 6, it is noted that a bore is drilled through the modulator body 50 and the excess length of it is blocked by a steel ball 51 press-fitted into the bore. An overview of the operation of this path will now be described. The pressure reduction valve 74 (Fig. 10) includes a one-way valve 114 (as well as the isolation valve 112) and the reapplication valve 68 includes a one-way valve 116 (Fig. 9). Both of the solenoid controlled valves 68 and 74 have two positions, that is, an unenergized position in which the respective one-way valve is held open and an energized position in which it is not held open but is allowed to open and close as controlled by the fluid pressure on each side of the one-way valve. Thus, the two one-way valves have two positions including a normally open position and a oneway operating position. The isolation valve 112 is closed when the pressure reduction valve 74 is unenergized and is open when the valve 74 is energized. In addition, it is noted that each of the one-way valves 114 and 116 (when not held open) block fluid flow in the braking direction (from the master cylinder reservoir 32 to the brakes 16-19), and allow fluid flow in the opposite, or pressure release direction.

Thus, it will be seen that in the normal unenergized condition of the modulator 10, there is an open passageway through the modulator allowing free flow of fluid therethrough and that the reciprocating operation of the piston 82 has no affect on this passageway. However, when the solenoids of the valves 68 and 74 are energized. the passageway through the modulator 10 is now defined between two one-wav valves and has an expansible and contractible volume, whereby fluid is. pumped from a wheel brake back to the master cylinder. After the pressure in the brake has been reduced, the valves 74 and 68 can be de-energized returning the modulator to its normal off condition allowing free flow of fluid therethrough. There are other states or conditions the modulator can have, which will be described below.

The piston 82 reciprocates during pumping, i.e. when the isolation valve 112 is opened. When the isolation valve is closed, the upward movement of the piston can force fluid out of the chamber 106 past the isolation valve, how ever, since no fluid can get into the chamber 106 past the closed isolation valve, there is no force to push the piston down as the cam rotates and the piston will remain up (away from the cam) until the next time the isolation valve is opened.

The individual reapplication valve 68 and the pressure reduc_tion valve 74 will now be described in detail with reference to the enlarged views thereof in Figs. 9 and 10, respectively. The reapplication valve 68 will now be described with reference to Figs. 9 and 12 which actually show the reapplication valve 64, however, they are identical so a description of one is a description of the other. Fig. 9 shows the valve 64 in its open or unenergized state, Fig. 12 shows the valve 64 in its unenergized but partly open state (following the pressure reduction cycle described below). The reapplication valve 64 includes a solenoid coil 120 and a fluted armature plunger 122 which moves to the left in Fig. 9 against the action of a return spring 124 when the solenoid coil 120 is energized. The reapplication valve 64 also includes a washer or valve disc member 128, a cooperating seat 130, a pin or seal element 132 and its biasing spring 134 fitting inside of a tubular member 136. The tubular member 136 is located in a bore 140 in a disc 142. The member 136 includes openings therethrough to accommodate fluid flow around the seal element 132. When the reapplication valve 64 is in its normal unenergized state, it allows free flow of fluid through the reapplication valve 64 through the tubular member 136, around the seal element 132, around and through the valve disc member 12S (through openings 143 in the edge thereof and through a central opening therein, that is, through an annular space 145 (Fig. 12) between the seal element 132 and the central opening in member 128), through flutes 144 in the plunger 122, out through a passage 146 and into a passageway (not shown but corresponding to passageway 104 in Fig. 7). The reapplication valve 64 also includes means for controlling the rate of rise of increasing fluid pressure applied through the modulator, similar to that described in U.S. Patent 4,138,165 (incorporated herein by reference). That is, the sealing element 132 and associated spring operate similar to that described in that patent. Thus, when the solenoid coil 120 is de-energized, the spring 124 can move the plunger 122 to the right holding the one-way valve 116 open. However, if the pressure differential is sufficiently high, the force of the return spring 124 is incapable of overcoming the pressure forces and the plunger 122 will only move part-way, far enough to unseat the sealing element 132 but not the valve disc member 128. With only the sealing element 132 unseated, a restricted flow path through the reapplication valve 64 is opened thus controlling the rate of rise of increasing fluid pressure applied through the modulator, until such time as the pressure differential diminishes allowing full movement of the plunger 122 which will also unseat valve disc member 128 and allow full flow of fluid through the reapplication valve 64.

The pressure reduction valve 74 will now be described in detail with reference to Fig. 10. The valve 74 includes a solenoid coil 150 and a fluted armature plunger 152 which moves to the left in Fig. 10 against the action of a return spring 154 when the solenoid coil 150 is energized in Fig. 10. The plunger 152 is hollow and includes a pin 156 press-fitted therein. The isolation valve 112 is a ball 157 press-fitted in the hollow plunger and adapted to seal against a seat 158 when the valve 74 is unenergized, thus sealing the chamber 106 from the passageway between the two one-way valves 114 and 116. The one-way valve 114 includes a ball 160, a seat 161 and a spring 162. The ball 160 is not press-fitted in the hollow plunger 152 but is slidably movable therein. When the plunger 152 is in its energized position (to the left in Fig. 10) the ball 160 can move against the spring 162 to open and close depending on the fluid pressure across the ball. When the plunger 152 is in its unenergized position (to the right in Fig. 10) the ball is held off of its seat 161 and the one-way valve 114 is held open. It will thus be seen that, with respect, for example, to wheel 13, the modulator 10 (see Fig. 1) is interposed in a single brake fluid conduit between the reservoir 32 and the brake 17 and in the normal operation of the vehicle has a passageway therethrough that allows the free flow of brake fluid therethrough. However, when the logic system 20 senses an impending wheel lock, the modulator 10 is caused to go into its pressure reduction state or condition by the logic system 20 energizing the solenoid valves 68 and 74, thus positioning the valves 114 and 116 for one-way valve operation and opening the isolation valve 112. This causes brake fluid to be pumped from the brake 17 back to the reservoir 32. When the logic system senses that wheel lock is no longer imminent, the modulator 10 is caused to go from its pressure reduction state to its pressure hold state by the logic system de-energizing only the solenoid in the valve 74, thus again isolating the expandible-contractible chamber 106 from the passageway between the one-way valves 114 and 116, while leaving valve 114 open and valve 116 closed. This prevents further pressure increase because valve 116 is closed and prevents further pressure decrease because valve 112 is closed. At the appropriate time, the logic system then causes the modulator to go from its pressure hold state to its pressure reapplication state by the logic system now de-energizing the solenoid in valve 68, whereby the modulator is returned to its original condition, with the exception that depending upon the pressure differential as discussed above, the valve 68 may not open all the way, thus controlling the rate of rise of pressure reapplication.

The sequence of modulation state- pressure reduction, pressure hold and pressure reapplication are not limited to the order described above; other sequences may be required by the logic system and followed by the modulator, e.g., pressure hold, pressure reduce and pressure reapply. Thus, this invention provides a single, compact modulator having only a single motor, for use for example, in automobiles, and which can operate the various brake conduits independently of each other. For example, the fluid pressure can be reduced at one wheel while it is being re applied at another wheel, with the single motor running all the time, and without the one action disturbing the other.

The above detailed description is only of the presently preferred embodiment of this invention. Other embodiments can be used. For example, it may be desired to have only three outlets from the modulator with a single fluid conduit to the rear wheels and separate conduits to each of the front wheels. Further, if desired a plurality of individual modulators can be used, each with a single pair of solenoid controlled one-way valves and a single isolation valve and a single piston. The isolation valve can be separate from the two one-way valves and can have its own solenoid control. The means for controlling the rate of rise of re-applying fluid pressure can be omitted if desired. The following terms are hereby defined for use inthe specification and claims: the term "normal or non antilock brake operation" means the periods of tine during which the anti-lock modulator 10 is not being used, that is, during non-braking operation as well as braking operation that does not use the anti-lock modulator; the one-way valves in the reapplication valves 62-68 will be referred to as the "upstream" valve and the other one-way valves in the pressure reduction valves 70-76 as the "downstream" valves; and the direction of brake fluid flow from the reservoir 32 to the brakes 16-19 is termed the "braking direction" and the opposite direction is termed the "pressure release direction".

The invention has been described in detail with particular reference to the preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

Claims

I claim: 1. Apparatus for braking a rotating member comprising hydraulic fluid pressure actuated braking means for braking rotation of the member; a reservoir for supplying fluid to said braking means, a single hydraulic fluid conduit connecting said reservoir to said braking means, means operatively communicating with the single conduit for applying therethrough hydraulic fluid pressure for braking the wheel, and anti-lock brake modulator means operatively interposed in said single conduit and including a pass- ageway therethrough for providing free flow of fluid therethrough in both a braking direction and a pressure release direction during non-anti-lock operation of said modulator, said modulator including: (1) means including a motor for pumping hydraulic fluid through said passageway and said single conduit in a pressure release direction away from said braking means and toward said pressure applying means for releasing braking force, and (2) means for allowing continuous operation of said motor while controllably isolating said pumping means from said passageway during non-pump ing operation.

2. The apparatus as recited in claim 1 wherein said isolating means include a solenoid controlled isolation valve.

3. The apparatus as recited in claim 2 wherein said pumping means includes a pair of one-way valves defin ing said passageway therebetween and including an expansible and contractible chamber in fluid communication with said passageway through said isolation valve.

4. The apparatus as recited in claim 3 including a first solenoid for operating oneof said one-way valves and a second solenoid for operating the other of said one-way valves and also said isolation valve.

5. The apparatus as recited in claim 3 wherein each of said one-way valves is a solenoid controlled valve.

6. The apparatus as recited in claim 5 wherein each of said one-way valves includes means for holding said one-way valves in an open position during non-anti-lock operation.

7. The apparatus as recited in claim 6 wherein said isolation valve is part of and is controlled with oneof said one-way valves.

8. Tne apparatus as recited in claim 7 wherein said one of said one-way valves is the downstream one-way valve.

9. The apparatus as recited in claim 8 wherein the upstream one of said one-way valves includes means for controlling the rate of pressure reapplication to said braking means.

10. The apparatus as recited in claim 9 wherein said rotating member is a set of four automotive vehicle wheels and wherein said modulator includes four sets of said pumping means and of said isolating means.

11. The apparatus as recited in claim 10 wherein said braking means includes a set of four wheel braking means and said modulator includes four outlets, each one connected to a different one each of said four wheel braking means.

12. The apparatus as recited in claim 11 wherein each of said sets of said pumping means and of said isolating means includes means for holding the brake pressure in said passageway substantially constant following pressure reduction by said pumping means.

13. The apparatus as recited in claim 1 wherein said modulator includes means for holding the brake pressure in said passageway substantially constant following pressure reduction by said pumping means.

14. The apparatus as recited in claim 1 wherein said modulator also includes means for controlling the rate of pressure reapplication to the braking means following pressure reduction or pressure holding.

15. Tne apparatus as recited in claim 1 further comprising logic means for signaling the occurrence of excessive slip of said rotating member and for energizing said pumping means.

16. An anti-lock brake modulator for use in a fluid conduit between a brake master cylinder reservoir and a rotating member braking means comprising:

(a) a passageway therethrough for providing free flow of fluid therethrough in both a braking direction and a pressure release direction during non-anti-lock brake operation of said modulator;

(b) means, including a motor, for pumping brake fluid through said passageway in the pressure release direction, and

(c) means for controllably isolating said pumping means from said passageway during non- pumping operation of- said modulator while allowing continuous operation of said motor.

17. The modulator as recited in claim 16 wherein said isolating means include a solenoid controlled isolation valve.

18. The modulator as recited in claim 17 wherein said pumping means includes a pair of one way valves defin ing said passageway therebetween and including an expansible and contractible chamber in fluid communication with said passageway through said isolation valve.

19. The modulator as recited in claim 18 wherein including a first solenoid for operating one of said one- way valves and a second solenoid for operating the other of said one-way valves and also said isolation valve.

20. The modulator as recited in claim 18 wherein each of said one-way valves is a solenoid controlled valve.

21. The modulator as recited in claim 20 wherein each of said one-way valves includes means for. holding said one-way valves in an open position during non-anti-lock operation.

22. The modulator as recited in claim 21 wherein said isolation valve is part of and is controlled with one of said one-way valves.

23. The modulator as recited in claim 22 wherein said one one-way valve is the downstream one-way valve.

24. The modulator as recited in claim 23 wherein the upstream one of said one-way valves includes means for controlling the rate of pressure reapplication to said braking means.

25. The modulator as recited in claim 24 wherein said rotating member is a sat of four automotive vehicle wheel and wherein said modulator includes four sets of said pumping means and of said isolating means.

26. The modulator as recited in claim 25 wherein said modulator includes four separate outlets.

27. The modulator as recited in claim 26 wherein each of said sets of said pumping means and of said isolating means includes means for holding the brake pressure in said passageway substantially constant following pressure reduction by said pumping means.

28. The modulator as recited in claim 16 wherein each of said sets of said pumping means and of said isolating means includes means for holding the brake pressure in said passageway substantially constant following pressure reduction by said pumping means.

29. The modulator as recited in claim 16 wherein said modulator also includes means for controlling the rate of pressure reapplication to the braking means following pressure reduction by said pumping means.

30. The modulator as recited in claim 16 wherein said modulator includes a plurality of individual, separate sets of said pumping means and of said isolating means.

31. The modulator as recited in claim 30 wherein said modulator includes a single motor and cam and a plurality of separate cam operated pistons, one for each of said sets, and wherein each of said chambers includes one of said pistons.

32. The modulator as recited in claim 31 wherein each of said sets includes means for controlling the rate of pressure reapplication to the braking means following pressure reduction by said pumping means.

33. The modulator as recited in claim 32 wherein each of said sets includes means for holding the brake pressure in said passageway substantially constant following pressure reduction by said pumping means.

34. The modulator as recited in claim 16 wherein each of said sets includes means for holding the brake pressure in said passageway substantially constant following pressure reduction by said pumping means.

35. An anti-lock brake modulator for use in an automotive vehicle brake fluid conduit between a brake master cylinder reservoir and the wheel braking means, said modulator including: (a) a plurality of substantially identical, independent passageways therethrough, each passageway providing free flow of fluid therethrough in both a braking direction and a pressure release direction;

(b) a single motor and cam and a plurality of pistons adapted to be reciprocatingly moved by said cam;

(c) a plurality of expansible and cqntractible chambers, each associated with one of said pistons;

(d) each of said passageways including means for pumping fluid in a pressure release direction therethrough, said pumping means including a pair of one-way valves defining said passage way therebetween;

(e) an isolation valve in each of said passageways for controlling fluid communication between said passageway and one each of said chambers for allowing continued operation of said motor while controllably isolating said pumping means from said passageway during non-pumping operation.

36. The modulator as recited in claim 35 including a solenoid for operating both the downstream one of said one-way valves and said isolation valve, and a separate solenoid for operating the upstream one of said one-way valves.

37. The modulator as recited in claim 35 including means for holding the pressure substantially constant in each passageway following fluid pressure reduction by the pumping means for that passageway.

38. The modulator as recited in claim 35 including means, in each passageway, for controlling the rate of pressure reapplication following pressure reduction in that passageway.

39. In a method of braking an automotive vehicle wheel wherein pressurized hydraulic fluid is supplied from a pressure source and applied through a single conduit to a wheel brake, and wherein an anti-lock modulator is inter- posed in said conduit and includes a pair of normally open, spaced-apart, controllable one-way valves defining a passageway therebetween and also includes an expansible and con tractible chamber in fluid communication with said passageway for pumping hydraulic fluid through the single conduit in a pressure release direction away from the brake and toward the source for reducing braking fprce when said oneway valves are allowed to close, the improvement comprising controllably isolating said chamber from said passageway except when pumping is desired.

40. The method as recited in claim 39 including sensing wheel slip, opening said isolation valve, moving said one-way valves from their normally open position to their one-way operating position, and pumping fluid in the pressure release direction through said passageway.

41. The method as recited in claim 39 wherein said pumping step comprises reciprocatingly moving a piston in said chamber with a motor and cam while said chamber is in fluid communication with said passageway, and wherein said isolating step comprises closing a solenoid controlled isolation valve between said chamber and said passageway whereby said motor can continue to operate without accomplishing any pumping in said passageway.

42. The method as recited in claim 41 including energizing said solenoid controlled isolation valve to open it and de-energizing it to close it.

43. The method as recited in claim 39 including holding the pressure substantially constant in said passageway following pressure reduction in the passageway.

44. The method as recited in claim 39 including controlling the rate of pressure reapplication in said passageway following pressure reduction in the passageway.

45. In a method for avoiding locking of a rotating member braked by force exerted by a fluid pressure activated cylinder wherein fluid is normally freely passed between the cylinder and a reservoir through a single conduit which establishes fluid flow communication therebetween and through a controllable anti-lock modulator interposed in said single conduit and having a passageway therethrough in fluid communication with said single conduit, and wherein a sensor senses occurrence of a rate of retardation of the rotating member which exceeds a rate indicative of a tendency of the rotating member to lock, and wherein the modulator responds to the sensed occurrence of an excessive rate of retardation by blocking free passage of fluid in a braking direction through said modulator and also by pumping fluid through the single conduit in the opposite pressure release direction for releasing braking force retarding rotation of the rotating member, and wherein said pumping comprises alternately expanding and contracting the volume of said passageway located between a pair of spaced apart, controllable, one-way valves by a motor and cam driving a piston reciprocatingly in a chamber in fluid communication with said passageway portion, the improvement comprising controllably isolating said chamber from said passageway during non-pumping operation of said passageway.

46. The method as recited in claim 45 including operating said motor and cam continuously.

47. The method as recited in claim 46 wherein said isolating step comprises positioning a solenoid controlled isolation valve between said chamber and said passageway.

48. The method as recited in claim 45 including providing said modulator with four separate sets of said passageways, said pair of one-way valves, and said piston and chamber, providing said modulator with a single motor and cam operating a plurality of pistons, one for each chamber, and operating said motor and cam continuously while separately and independently controlling fluid flow through each of said passageways.

49. The method as recited in claim 45 including controlling said isolation valve and one of said one-way valves with a single solenoid.

50. The method as recited in claim 45 including holding the pressure substantially constant in each passageway following pressure reduction in that passageway.

51. The method as recited in claim 45 including controlling the rate of pressure reapplication in each passageway following pressure reduction in that passageway.