US 4,133,178 AGrant
Quick Take-Up Master Cylinder
Issue Date:1979-01-09
•4 Claims
•2 Drawing Sheets
Abstract
A master cylinder assembly has a low pressure, high volume displacement quick take-up chamber and a high pressure, low volume displacement pressurizing chamber formed by a stepped bore and a stepped piston. A compensation control and blow-off valve unit has a peripheral lip seal type valve providing compensation flow on break release but preventing flow from the quick take-up chamber during brake apply. A normally closed check valve is held open while the master cylinder is in the fully released position to provide communication between both pressurizing chambers and the fluid reservoir. As the pressurizing cup for the high pressure chamber closes its compensation port, the check valve is permitted to close by camming action of a valve pin and a camming surface on the piston. Initial flow is obtained from the quick take-up chamber past the pressurizing cup into the high pressure chamber. At a predetermined pressure level, arranged to be sufficient to take up lining clearance and initially apply the vehicle brakes, the pressure in the quick take-up chamber opens the normally closed check valve so that fluid from the quick take-up chamber then flows to the reservoir without further pressure buildup in that chamber. As the master cylinder is released, compensation fluid can flow past the peripheral compensation valve to both chambers. When the master cylinder is returned to the fully released position, the check valve is forced open by action of the piston cam surface and the valve pin to provide fluid communication between the reservoir and both pressurizing chambers. A fluid flow restrictive orifice is provided in series with the normally closed check valve to prevent a high rate of pressure increase in the quick take-up chamber from being bypassed fully to the fluid reservoir upon a high rate of master cylinder actuation, forcing at least some of the pressure to pass by the cup seal into the high pressure chamber and the brake apply circuit.
Metadata
Assignee
- General Motors Corporation
Inventor
- Frank W. Brooks, Sr.
Application Information
Application Number:US 7886276
Filing Date:1977-04-18
Priority Date:1977-04-18
Art Unit:341
Classifications
IPC:
B60T 1108F15B 704
Field of Search:
60137251562;574;578;581;585;588;589;592523;512.1;512.3;493.883
Patent Drawings (2 sheets)
Description
The invention relates to a quick take-up master cylinder and more particularly to one having a compensation control and blow-off valve unit.
It is a feature of the invention to utilize a blow-off valve which permits the buildup of quick take-up pressure to a predetermined point after which the quick take-up chamber is connected to the master cylinder reservoir and the blow-off pressure reduces to substantially, for example to approximately one-half of its original value. This reduces the portion of the master cylinder actuating force required during the remainder of the stroke to force fluid in the quick take-up chamber through the blow-off valve.
It is another feature of the invention to provide a restrictive fluid flow orifice in series with the blow-off valve to provide constant brake pedal travel regardless of the rate of brake pedal apply. Fluid from the quick take-up chamber must pass around the primary pressurizing cup into the high pressure chamber and the brake circuit leading to the vehicle brakes for quick take-up. If the brake pedal is applied at an extremely fast rate, such as may occur when the vehicle operator applies full braking force by jamming the brake pedal instead of applying a modulated force, the fluid from the quick take-up chamber may tend to flow primarily into the reservoir rather than into the high pressure pressurizing chamber if the restrictive flow orifice is not provided. Therefore the orifice assures a reasonable flow of quick take-up fluid into the brake system under such circumstances.
The quick take-up master cylinder takes up excessive pedal travel by use of a two-stage bore and piston arrangement. It permits the use of increased brake lining clearances, particularly in disc brakes, thus improving gas mileage. It reduces brake pedal effort to manually actuate brakes by using a smaller master cylinder bore diameter for generating high braking pressures after the brake linings have engaged the brake discs or drums. This can remove the requirement of power brake boosters for some cars.
The master cylinder has a stepped bore and a stepped piston defining a large volume chamber and a smaller volume chamber. The large chamber is effective at the beginning of the stroke and in the lower pressure range. It displaces a large volume of fluid at relatively low pressure, which takes up brake lining clearances with a relatively small amount of brake pedal travel. Brake fluid is forced from the large volume or quick take-up chamber around the lip of the primary piston seal into the high pressure chamber as the stroke of the master cylinder piston increases in the brake apply direction. Fluid pressure builds up in the quick take-up chamber to a point that causes the blow-off valve to open and permit passage of fluid from the quick take-up chamber to the master cylinder reservoir. During the return stroke of the master cylinder piston, excess fluid that has passed from the quick take-up chamber is restored by the flow of fluid from the reservoir around the lip of a compensating seal valve. The blow-off valve is held in the open position when the master cylinder is in the fully released or at-rest position, permitting free passage of brake fluid from the high pressure chamber and the quick take-up chamber into the master cylinder reservoir.
In the Drawings
FIG. 1 is a side view of a master cylinder embodying the invention, with parts broken away and in section. The master cylinder is illustrated as being in the fully released position.
FIG. 2 is an enlarged view of the compensation control and blow-off unit of the master cylinder of FIG. 1, with parts broken away and in section.
FIG. 3 is a view of the master cylinder of FIG. 1, and similar to that Figure, showing the master cylinder in the initial brake applying mode before the blow-off valve has been actuated.
FIG. 4 is similar to FIG. 1 and shows the master cylinder during brake apply immediately after the blow-off pressure has been attained.
FIG. 5 is similar to FIG. 1 and shows the master cylinder in the process of release.
The master cylinder assembly 10 includes a housing 12 and a reservoir body 14. The housing has a stepped bore 16 formed therein with a smaller diameter forward section 18 and a larger diameter rear section 20. Since the master cylinder is shown as being of the dual circuit type, it is provided with two pressurizing pistons. The forward piston 22 is the secondary piston and is reciprocably received in the forward portion of bore section 18. The rear piston 24 is the primary piston and is a stepped piston. It has a forward piston head 26 formed as a piston land and reciprocably received in the rear portion of bore section 18. It has a rear piston head 28 formed as a land and reciprocably received in bore section 20. The piston heads 26 and 28 are joined by a reduced diameter section 30.
Piston 24 cooperates with bore 16 to define a high volume or quick take-up chamber 32 between piston heads 26 and 28. A high pressure pressurizing chamber 34 is defined by the rear part of bore section 18, piston head 26, and the rear end 36 of secondary piston 22. Another high pressure pressurizing chamber 38 is defined by the forward end 40 of secondary piston 22 and the forward end of bore section 18. An outlet 42 is provided in housing 12 to deliver brake fluid pressurized in chamber 34 to one brake circuit, and an outlet 44 is provided in housing 12 to deliver brake fluid from pressurizing chamber 38 to another brake circuit. Piston 22 has cup seals 46 and 48 respectively mounted on piston ends 36 and 40 in the usual manner. Cup seal 46 will prevent flow from chamber 34 past secondary piston end 36. Cup seal 48 will prevent fluid flow from pressurizing chamber 38 past piston end 40. The chamber 50 between the piston ends 36 and 40 is a compensation chamber which is continually connected to the brake fluid reservoir chamber 52 through compensation port 54. A compensation port 56 provides communication between pressurizing chamber 38 and reservoir chamber 52 when the secondary piston is in the fully released position shown in FIG. 1 so that cup seal 48 uncovers port 56. A piston return spring 58 is provided in chamber 38 and continually urges secondary piston 22 toward the released position. A piston return spring 60 in chamber 34 continually urges piston 24 toward the released position. Spring 60 is a caged spring, as is well-known in the art, which when extended to its permissible limit also establishes the release position of secondary piston 22.
A cup seal 62 is mounted on the forward side of piston head 26 of primary piston 24. Seal 62 is arranged to prevent fluid flow from pressurizing chamber 34 to quick take-up chamber 32, which acting as a check valve which permits fluid flow from chamber 32 to chamber 34 around the outer periphery of piston head 26 and the lip of cup seal 62 while pressure in chamber 32 is greater than pressure in chamber 34. A seal 64 is mounted on piston head 28 to seal the rear end of chamber 32. A piston stop 66 is provided in the open end of the enlarged bore section 20 and forms an abutment engaged by piston head 28 when the master cylinder is in the fully released position.
The reservoir body 14, in addition to fluid chamber 52 for the forward pressurizing chamber 38, has a fluid chamber 68 for the pressurizing chambers 32 and 34. The reservoir body is secured to the master cylinder housing by use of annular retaining and sealing means 70 and 72 which cooperate with short mounting tubes 74 and 76 formed as a part of reservoir body 14. The retaining and sealing means 70 and 72 and the mounting tubes 74 and 76 are respectively received in annular bosses 78 and 80 formed as a part of housing 12. Boss 78 has a recess 82 opening upwardly to receive retaining and sealing means 70 and mounting tube 74 and in open communication with chamber 52. Compensation ports 54 and 56 open through the bottom surface 84 of recess 82, as well as the side wall of bore section 18.
Boss 80 has a recess 86 which similarly receives retaining and sealing means 72 and mounting tube 76. The lower portion of recess 86 is also a valve unit chamber receiving the compensation control and blow-off valve unit 88. This portion of the master cylinder assembly is in greater detail in FIG. 2. The lower portion of recess 86 forming the valve unit chamber has a side wall 90, a bottom surface 92, a beveled annular shoulder 94 and an annular groove 96 adjacent and above shoulder 94. Compensation port 98 is formed through bottom surface 92 and opens into the rear end of bore section 18. It is so positioned relative to the fully released position of cup seal 62 that it opens into pressurizing chamber 34, but is closed during the initial brake apply movement of piston 24 by the lip of cup seal 62. Compensation port 100 extends through bottom surface 92 and into the larger diameter quick take-up chamber 32. Another port 102 extends through bottom surface 92 and into the rear end of bore section 18. Port 102 is located axially intermediate compensation ports 98 and 100 in relation to the axis of bore 16.
Unit 88 includes a valve body 104 having a passage 106 formed axially through it. The upper end of passage 106 is a restrictive flow orifice 108 which opens into reservoir chamber 68. The intermediate portion of passage 106 defines a valve chamber 110. The lower portion of passage 106 is closed by a valve seat member 112 secured to the lower end of valve body 104. Valve seat member 112 has a passage 114 extending therethrough with the upper end beveled to provide a valve seat 116. The lower end opens into a lower chamber 118 defined by recessed bottom surface 92, the lower portion of recessed side wall 90, and the undersurface of valve seat member 112. The valve seat member may be provided with castellated supports 120 to maintain chamber 118. A flange 122 around the center part of valve body 104 is beveled to mate with shoulder 94 to axially and radially locate the valve body in the lower portion of recess 86. Retainer 124 fits in groove 96 and over flange 122 to hold the valve body in position against axial movement. A series of circumferentially spaced passages 126 are formed to extend axially through flange 122 and generally axially parallel to the axis of passage 106. A cup seal 128 is peripherally mounted on the lower portion of valve body 104, which is similar in diameter than the diameter of side wall 90. Seal 128 has a lip 130 which is sealingly engageable with recess side wall 90. Passages 126 connect the top portion 132 of the valve unit chamber of recess 86 with the upper side of cup seal 128 so that fluid may flow from chamber 68 through passages 126 and past cup seal lip 130 into chamber 118. Thus the cup seal acts as a valve permitting such flow while preventing flow in the reverse direction, and is a lip type compensating seal.
A poppet type valve 134 is positioned for axial movement in valve chamber 110 of passage 106 of valve body 104. The valve 134 has a lower surface or side 136 which is engageable with valve seat 116 to close the lower end of passage 106. A spring 138 provides yieldable means urging valve 134 toward valve seat 116. One end of spring 138 seats on the upper side 140 of valve 134 and the other end of spring 138 seats on an annular spring seat 142 provided in the upper end of valve chamber 110 so as not to interfere with orifice 108. A valve pin 144, which may be integrally formed with valve 134 or separately provided and engageable with that valve, extends downwardly through passage 114, chamber 118, and port 102 into the rear end of bore section 18. Passage 114 is sufficiently larger than pin 144 to permit fluid flow therethrough around the pin. Port 102 permits guided reciprocable movement of valve pin 144 but permits no appreciable flow between the pin and the port side wall. The lower end 146 of valve pin 144 is arranged to engage cam surface 148, formed on piston head 26 of primary piston 24, under certain conditions. Cam surface 148 includes a surface 150 immediately adjacent cup seal 62 and axially parallel to the axis of the piston 24, and an angled surface 152 providing a beveled shoulder-like arrangement from surface 150 to a smaller diameter surface 154. While pin 144 engages surface 150, it holds valve 134 open relative to valve seat 116 against the force of spring 138 urging the valve closed.
During brake apply movement of piston 24, pin 144 follows angled surface 152 radially toward surface 154, with spring 138 urging valve 134 and pin 144 downwardly until the valve side 136 engages valve seat 116 to close the valve. Under this condition valve pin end 146 is radially outward but slightly spaced from surface 154. Further movement of piston 24 in the brake applying direction does not cause any camming action of the valve pin. When piston 24 moves in a brake released direction, the valve pin end 146 engages angled surface 152 and is cammed upwardly so that the valve pin moves valve 134 away from its seat 116 in the fully released position. Following this, the valve pin end 146 again rests on surface 150 and the valve pin holds the valve 134 open.
The master cylinder assembly 10 is arranged to be actuated by a vehicle operator through a brake pedal 156 and a push rod 158 which engages the rear end 160 of primary piston 24. Upon initial brake applying movement of piston 24, the volume of quick take-up chamber 32 decreases and fluid is pushed past cup seal 62 into chamber 34. The cup seal 62 moves to close off compensation port 98 so that pressurization can be obtained in chamber 34. The initial movement of piston head 26 and cam surface 148 will permit valve pin 144 to move downwardly to permit closure of check valve 134. This prevents fluid flow between quick take-up chamber 32 and reservoir chamber 68 through passage 106. Thus pressurization of the fluid in chamber 32 can occur. This condition is shown in FIG. 3. Fluid is being displaced from chamber 32 at a relatively large volume but low pressure. It flows past cup seal 62, into chamber 34, and through outlet 42 to the brake circuit connected to that outlet. The pressure also acts on the rear end 36 of secondary piston 22 to move that piston forwardly so that its cup seal 48 closes compensation port 56 and fluid pressurization begins in chamber 38. Fluid so pressurized goes through outlet 44 to the brake circuit connected with that outlet. In a brake system where disc brakes are used on the front vehicle wheels only, the disc brake actuating circuit would be connected to outlet 42, thus taking advantage of the high volume of fluid being delivered through that outlet to provide for quick take-up of the brake linings so that they move quickly to engage the brake discs.
The pressure in chamber 32 acts on the lower surface 136 of valve 134, while the upper side 140 of the valve is exposed to reservoir pressure, which is usually substantially atmospheric pressure. As the pressure in chamber 32 increases, it reaches a point where the pressure differential acting on valve 134 overcomes the spring closing force of spring 138, moving the valve upwardly as seen in FIG. 4. This communicates the quick take-up chamber 32 with reservoir chamber 68 through passage 106, including restrictive flow orifice 108. Once the valve moves upwardly so that it disengages from its valve seat 116, the pressure from chamber 32 acts on a greater portion of the valve lower surface 136, thereby permitting a reduction in pressure in the quick take-up chamber 32 without permitting closure of valve 134. By proportioning the area of the valve exposed to quick take-up pressure when the valve is closed relative to the total valve lower surface area, quick take-up pressure is permitted to drop substantially, for example to approximately one-half of its blow-off pressure value, thereby substantially reducing the amount of brake pedal effort required to displace fluid from chamber 32 as the master cylinder is further actuated in the brake apply mode. Further master cylinder actuation causes pressures to be generated in chambers 34 and 38 at a high rate relative to the amount of stroke since the brake circuits no longer require any appreciable additional fluid volume. The pressurization then depends primarily on the brake pedal force exerted without requiring much brake pedal travel.
Upon brake release, pistons 22 and 24 move rightwardly as seen in the drawings, and more particularly as illustrated in FIG. 5. Any required compensating fluid for chamber 38 will flow through port 54 from reservoir chamber 52 to chamber 50 and past cup seal 48, as indicated by the arrows. At the same time, fluid is flowing from the brake circuit through outlet 44 into chamber 38. Fluid required for compensation of pressurizing chamber 34 will flow from reservoir chamber 68 through valve chamber top portion 132, passages 126, past lip 130 of cup seal 128, and through lower chamber 118 to compensation port 98. The fluid will then flow past cup seal 62 into chamber 34. Compensation fluid from the reservoir entering lower chamber 118 will also flow through compensation port 100 to quick take-up chamber 32. This chamber requires relatively large amounts of compensating fluid since the volume of chamber 32 increases rapidly upon brake release. The passages and chambers provided are sufficient for this purpose.
As the master cylinder reaches its fully released position, secondary piston cup seal 48 will open compensation port 56 so that pressurizing chamber 38 is directly connected through that port to reservoir chamber 52. Cup seal 62 will open compensation port 98 so that pressurizing chamber 34 is directly connected to lower chamber 118. By the time compensation port 98 is so opened, cam surface 148 has moved valve pin 144 upwardly to open passage 106 by upward movement of valve 134, thus providing direct communication between lower chamber 118 and reservoir chamber 68. Thus any temperature changes requiring fluid compensation after the brakes have been released are readily permitted since both pressurizing chambers 34 and 38 have open lines of fluid communication with the reservoir chambers of the reservoir body.
Under some conditions of operation, the vehicle operator may jam the brake pedal 156 in an attempt to obtain a very hard brake apply. Under such circumstances, the pressure increase in quick take-up chamber 32 may be such that the blow-off pressure is reached before sufficient volume can be pushed past cup seal 62 to the brake circuit served by chamber 34 and outlet 42. This causes the blow-off valve 134 to open and permit fluid flow from chamber 32 to reservoir chamber 68. To minimize the loss of fluid volume in this manner, restrictive flow orifice 108 is provided in series with the blow-off valve 134, restricting the amount of fluid flow that would otherwise occur and providing constant brake pedal travel irrespective of the rate of brake pedal apply.
Claims
What is claimed is:
1. In a quick take-up master cylinder having a fluid reservoir, a stepped bore, first and second compensation ports connected to said stepped bore, and a mating stepped piston reciprocably received in said bore, said piston and said bore defining a large diameter quick take-up pressurizing first chamber incontinuous fluid communication with said first compensation port; and a smaller diameter high pressure pressurizing second chamber, a compensation control and blow-off valve unit comprising: a first valve permitting fluid flow therepast from said reservoir through said first compensation port and to said first chamber and preventing fluid flow therepast in the reverse direction; a second valve having yieldable means continually urging said second valve closed, said second valve having one side exposed to fluid in said reservoir and another side exposed to fluid in said bore; and means for opening said second valve against the urging of said yieldable means and including a cam surface on said piston and a valve pin engageable with said cam surface, said cam surface engaging said pin to hold said second valve open when said piston is in the master cylinder fully released position and disengaging said pin to permit said yieldable means to close said second valve when said piston is moved in said bore away from the master cylinder fully released position to a pressurizing position; said second valve opening means further including an area of said second valve on said another side continuously exposed to fluid in said first chamber and responsive to a predetermined pressure differential between a higher fluid pressure generated in said first chamber during master cylinder actuation and the lower fluid pressure of said reservoir to open said second valve against the urging of said yieldable means to relieve pressure in said first chamber independently of action of said valve pin and cam surface; whereby pressure generated in said first chamber during master cylinder actuation is limited so that the master cylinder actuating force is not required to overcome pressure generated in said first chamber beyond that to provide quick take-up.
2. In a quick take-up master cylinder having a fluid reservoir, a stepped bore, first and second compensation ports connected to said stepped bore, and a mating stepped piston reciprocably received in said bore, said piston and said bore defining a large diameter quick take-up pressurizing first chamber incontinuous fluid communication with said first compensation port; and a smaller diameter high pressure pressurizing second chamber, a compensation control and blow-off valve unit comprising: a first valve permitting fluid flow therepast from said reservoir through said first compensation port and to said first chamber and preventing fluid flow therepast in the reverse direction; a second valve having yieldable means continually urging said second valve closed, said second valve having one side exposed to fluid in said reservoir and another side exposed to fluid in said bore; and means for opening said second valve against the urging of said yieldable means and including a cam surface on said piston and a valve pin engageable with said cam surface, said cam surface engaging said pin to hold said second valve open when said piston is in the master cylinder fully releasec position and disengaging said pin to permit said yieldable means to close said second valve when said piston is moved in said bore away from the master cylinder fully released position to a pressurizing position; said second valve opening means further including an area of said second valve on said another side continuously exposed to fluid in said first chamber and responsive to a predetermined pressure differential between a higher fluid pressure generated in said first chamber during master cylinder actuation and the lower fluid pressure of said reservoir to open said second valve against the urging of said yieldable means to relieve pressure in said first chamber independently of action of said valve pin and cam surface, said area of said second valve exposed to fluid in said bore effectively increasing upon opening of said second valve so that the pressure in said first chamber required to hold said second valve open is substantially less than the pressure in said first chamber required to open said second valve; whereby pressure generated in said first chamber during master cylinder actuation is limited so that the master cylinder actuating force for further brake actuation after said second valve is open is not required to overcome pressure generated in said first chamber beyond that which is substantially less than that pressure required to provide quick take-up.
3. In a quick take-up brake actuating master cylinder having a fluid reservoir, a stepped bore, first and second compensation ports connected to said stepped bore, a mating stepped piston reciprocably received in said bore and movable by a brake pedal, said piston and said bore defining a large diameter quick take-up pressurizing first chamber in continuous fluid communication with said first compensation port and a smaller diameter high pressure pressurizing second chamber, and check valve means on said piston permitting fluid flow from said first chamber to said second chamber, a compensation control and blow-off valve unit comprising: a first valve permitting fluid flow therepast from said reservoir through said first compensation port and to said first chamber and preventing fluid flow therepast in the reverse direction; a second valve having yieldable means continually urging said second valve closed, said second valve having one side exposed to fluid in said reservoir and another side exposed to fluid in said bore; a restrictive flow orifice fluidly intermediate said second valve one side and said reservoir through which any fluid flowing through said second valve must flow; and means for opening said second valve against the urging of said yieldable means and including a cam surface on said piston and a valve pin engageable with said cam surface, said cam surface engaging said pin to hold said second valve open when said piston is in the master cylinder fully released position and disengaging said pin to permit said yieldable means to close said second valve when said piston is moved in said bore away from the master cylinder fully released position to a pressurizing position, said second valve opening means further including an area of said second valve on said other side continuously exposed to fluid in said first chamber and responsive to a predetermined pressure differential between a higher fluid pressure generated in said first chamber during master cylinder actuation and the lower fluid pressure of said reservoir to open said second valve against the urging of said yieldable means to relieve pressure in said first chamber independently of action of said valve pin and cam surface at a maximum flow rate established by said restrictive flow orifice; whereby pressure generated in said first chamber during master cylinder actuation is limited so that the master cylinder actuating force is not required to overcome pressure generated in said first chamber beyond that to provide quick take-up, said maximum flow rate through said restrictive flow orifice controlling the rate of fluid displacement from said first chamber to said reservoir so that a fast and full force brake pedal actuation will direct at least a portion of fluid from said first chamber into said second chamber through said check valve means instead of directing all of it to said reservoir.
4. In a quick take-up master cylinder having a fluid reservoir, a master cylinder body with a stepped bore formed therein, and a mating stepped piston received in said bore and actuatable therein in apply and release modes, said bore and said piston defining a large diameter quick take-up pressurizing chamber and a smaller diameter high pressure pressurizing chamber, an improved compensation control and blow-off unit operable during the apply mode of said piston to selectively vent said quick take-up pressurizing chamber and comprising: a valve chamber formed in said master cylinder body with a side wall and a bottom surface and a top opening into said reservoir, said bottom surface having first and second compensation ports extending therethrough and into said bore, said first compensation port being continually connected with said quick take-up pressurizing chamber and said second compensation port being connected with said high pressure pressurizing chamber when said piston is in the fully released position of said release mode, and a third port extending from said bore through said bottom surface into said valve chamber; a valve assembly including a valve body having a lip type compensating seal extending about the outer periphery thereof and in sealing contact with said valve chamber side wall to permit fluid flow in one direction only, said valve body including means permitting fluid to pass in said one direction from said reservoir past said compensating seal to said compensation ports and said quick take-up pressurizing chamber as said piston is actuated in said release mode to said fully released position thereof; said valve body having a passage extending axially therethrough and connecting said reservoir and the portion of said valve chamber between said valve body and said valve chamber bottom surface and having a check valve therein which when closed prevents flow from said reservoir through said passage to said compensation ports when said piston is in either apply or release modes; and a pin having one portion extending through said third port into said bore for engaging said piston and another portion acting on said check valve to open or close said check valve in response to actuation of said piston, said piston having a cam surface engageable with said one pin portion to open said check valve when said piston approaches said fully release position of said release mode to permit fluid to pass from said high pressure pressurizing chamber to said reservoir through said passage, the cam surface of said piston disengaging from said one pin portion when said piston is initially actuated from said fully released position in said apply mode to permit closure of said check valve to retain fluid in said quick take-up pressurizing chamber, said check valve opening in response to a predetermined blow-off pressure generated in said quick take-up pressurizing chamber during further actuation of said piston from said fully released position in said apply mode to vent said quick take-up pressurizing chamber to said reservoir irrespective of engagement or disengagement of said one pin portion with said piston.
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