JPH0445972Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a master cylinder used in a brake device of a vehicle or the like. [Prior art and its problems] In a master cylinder, a valve device called a so-called center valve is disposed on a piston inserted into a cylinder hole, and a valve device called a center valve is disposed on a piston inserted into a cylinder hole. There is a valve device that controls the supply to. FIG. 8 shows this type of brake master cylinder disclosed in Japanese Patent Laid-Open No. 142743/1983. In this brake master cylinder 1, a center valve type valve device is housed in a piston 4 that is slidably inserted into a cylinder hole 16. This valve device consists of a valve member 8, a valve rod 9, a spring 10, etc., and the valve member 8 and spring 10 are connected to a piston 4.
The valve rod 9 is arranged in a central through-bore 11 formed in the piston. One end of the valve rod 9 passes through the piston 4 and is connected to the hole 1.
The piston is inserted into a hole 12 of the piston extending in a direction intersecting with the piston. A pin 13 is provided in the hole 12, and the pin 13 is connected to a circlip 1 that is movable in the circumferential direction on the circumferential surface of the piston 4, as shown in FIG.
4. One end of the valve rod 9 is brought into contact with a pin 13 by the biasing force of a spring 10, so that its range of movement is restricted. In the brake master cylinder 1 equipped with such a valve device, when the push rod 3 is operated, the piston 4 moves in the same direction as the push rod 3, and this operation causes the spring 10 to close the valve member 8. At this time, the circlip 14 moves on the circumferential surface of the piston 4, but is prevented from further movement by coming into contact with a stop washer 15 fixed to the cylinder housing 2. By the way, in the brake master cylinder 1, the hydraulic fluid supplied from a reservoir (not shown) is as follows.
The hydraulic fluid is supplied into the pressure chamber 6 through the adjustment hole 5, the hole 12, the hole 11, and the recess 7, but in order to set the master cylinder 1 having such a fluid path to the use position of the vehicle, the hydraulic fluid is It is necessary to fill it in advance. In this case, the air in the cylinder housing 2 is normally suctioned by a vacuum pump through the adjustment hole 5 and the liquid connection port 18, and from this state, the hydraulic fluid is sucked into the cylinder. However, in this type of master cylinder 1, during the air bleeding process, that is, the vacuum bleeding process, the guide member 17 that guides the sliding movement of the piston 4 is exposed to the differential pressure between the suction side pressure and the atmospheric pressure. Therefore, it receives an acting force that moves forward toward the gap formed on the lock washer 15 side. For this reason, in the conventional one shown in the figure, a stop washer 15 is fixed to an annular groove formed at the back of the cylinder hole of the housing 2, and the guide member 17 is prevented from moving toward the gap side by the stop washer 15. I try to do it, but
The work of fixing the stop washer 15 in the annular groove becomes extremely difficult. Therefore, if an attempt is made to eliminate the need for the fixing work, the following problems arise. That is, the lock washer 1 is formed without forming the annular groove.
5 is simply inserted into the cylinder hole like the guide member 17, or the lock washer 15
If this is omitted, the guide member 17 receives the differential pressure during vacuum bleeding, moves forward greatly without being prevented from moving, and the circlip 1
4 is pressed so that its front end comes into contact with the piston 4, thereby causing the piston 4 to advance through the circlip 14. At this time, the valve member 8
Although not shown, a slave piston is disposed in front of the piston 4, and since this is also provided with a valve device configured as a center valve, the piston 4 is kept in an open state. The slave piston is moved forward by compressing a spring 6 that is stretched between the piston and the slave piston (not shown) located in front of the piston. Thereby, the valve member of the valve device provided in the slave piston is seated on the valve seat formed on the inner wall of the piston. That is, the valve device of the slave piston is closed. Therefore, it becomes difficult to bleed air in this state. Therefore, in that case, it is necessary to open the valve device again by manual operation or the like to remove the remaining air from the pressure chamber 6 side, which poses a problem in that it takes time and effort. [Problems to be solved by the invention] The present invention was made in view of the above-mentioned circumstances, and aims to reliably bleed air without deteriorating workability during assembly and without requiring extra effort during vacuum bleeding. The purpose is to provide a master cylinder that can [Means for Solving the Problems] The above objective is to provide a cylinder body having a cylinder hole, a shaft portion that is slidably inserted into the cylinder hole and extends to the outside of the cylinder body, and a shaft portion that is larger than the shaft portion. a main piston having a head that slides in the cylinder hole with a diameter, a slave piston that is slidably inserted into the cylinder hole, and a main pressure chamber formed between the main piston and the slave piston. and a slave pressure chamber formed between the bottom of the cylinder hole and the slave piston, and a slave pressure chamber formed in each piston so as to communicate the main pressure chamber and the slave pressure chamber to a hydraulic fluid reservoir side, respectively. a fluid passage provided in each of these fluid passages to restrict communication between the main pressure chamber and the slave pressure chamber and the hydraulic fluid reservoir; a valve device having a valve member biased toward a valve seat provided on the piston by a valve spring, the valve device being configured to close in response to movement of the piston toward a pressure chamber; A guide member that guides the sliding movement of the shaft of the main piston at the opening of the cylinder hole, and a sealing member that seals the cylinder hole, the shaft of the main piston, and the guide member at the opening of the cylinder hole. a retaining member provided at the opening of the cylinder hole to prevent the guide member from moving outside the cylinder hole; and a retaining member provided at the opening of the cylinder hole to prevent the guide member from moving outside the cylinder hole; An annular member fitted to the outer periphery of the shaft portion of the main piston and capable of abutting against the guide member; a main valve opening means for opening the valve device of the main piston, the main valve opening means having a shaft-shaped member that can be engaged with so as to separate the main piston from the valve seat; engages with the valve member from the valve seat side until it moves a predetermined amount toward the slave pressure chamber side;
A master cylinder comprising a slave opening means for opening the valve device of the slave piston by disengaging the valve member from the valve seat against the biasing force of the spring, wherein the guide member A master cylinder provided with a spring member that biases the guide member toward the retaining member by a biasing force that overcomes the differential pressure when the pressure acting on the inside of the cylinder hole becomes lower than the pressure acting on the outside thereof. achieved by. [Operation] According to the above configuration, even if the guide member tries to advance inward during vacuum bleeding, the guide member is pressed against the retaining member by the spring force from the spring member without separating from the retaining member. maintain the condition. Therefore, the valve device of the slave piston remains in its original state without being closed. [Embodiment] Hereinafter, a master cylinder according to an embodiment of the present invention will be described with reference to FIGS. 1 to 6. In the figure, the master cylinder of this embodiment is indicated as a whole by 21. A stepped cylinder hole 23 is provided in the cylinder body 22 of the master cylinder 21.
The front end (left side in the figure) is a closed end, and the rear end (right side in the figure) opening is closed by a guide member 24, which will be described later. (Note that in this specification, the front means the left side of the figure, and the rear means the right side of the figure.) The guide member 24 has a seal ring 26 interposed therebetween by a stop ring 25 as a retaining member. and is fixed to the cylinder body 22. A shaft portion 28 of a main piston 28 with a cup seal 27 attached thereto is inserted into the center through hole 24a of the guide member 24.
a is slidably fitted. Further, a recess 28b formed in the shaft portion 28a of the main piston 28 receives a tip end of an output shaft of a negative pressure booster (not shown). The cylinder hole 23 of the cylinder body 22 consists of a large-diameter hole 23a into which the guide member 24 is fitted and a small-diameter hole 23b that occupies most of the part. A slave piston 29 is slidably fitted into the left side of the small diameter hole 23b. The main piston 28 has a large diameter portion 28c at its front end, and a cup seal 30 is mounted thereon. Further, the left side portion of the main piston 28 is formed as a cylindrical portion 28d. A hole 45 is formed in the peripheral wall of the cylindrical portion 28d,
As a result, the inner hole of the cylindrical portion 28d and both pistons 2
Liquid communication is performed with a main pressure chamber 46 formed between 8 and 29. A spring receiver 41 and a stop ring 42 are provided at the open end of the cylindrical portion 28d. A spring 34 is stretched between the spring receiver 41 and a valve rubber 33 attached to the tip of the valve rod 32, and urges the valve rod 32 to the right in the figure. The valve rod 32 is slidably fitted into an axial through hole 35 formed in the main piston 28. The rear end of the valve rod 32 is inserted into a bore 36 in the piston 28 that extends through the main piston 28 and in a direction transverse to the axial bore 35. A pin 37 is disposed within the bore 36, the pin 37 passing radially through the main piston 28 and having a step 38a at one end for receiving a spring, as shown in FIG.
Both ends thereof are firmly supported by an annular member 38 provided with. The annular member 38 guides the pin 37 movably in the axial direction within the hole 36, and the guide member 2
The rearward movement is restricted by a support member 39 disposed in front of 4. That is, in this embodiment, the annular member 38 is in contact with the guide member 24 via the support member 39. Additionally, the annular member 38
is brought into contact with the support member 39 by the biasing force of a spring 40 stretched between the step portion 38a of the member 38 and the flange portion of the main piston 28. Further, the rear end of the valve rod 32 is brought into contact with the pin 37 by the biasing force of the spring 34, so that rearward movement is restricted and is brought into contact with the pin 37. In addition, in this brake master cylinder 1, as shown in an enlarged view in FIG. 4, a wave washer shown in FIGS. 88 is interposed, and the wave washer 88 functions as a spring member. The guide member 24 is urged toward the open end side of the cylinder hole 23 by the pressing force generated by the twisted posture of the wave washer 88, but is stopped by the stop ring 25.
It has been received in a state where there is no rattling or movement. On the other hand, the bottom wall portion of the cylindrical portion 28d of the piston 28 functions as a valve seat 43, and normally faces the end surface of the valve rubber 33 with a predetermined gap as shown in the figure. Such valve rubber 33, valve seat 43, valve rod 3
2. The spring 34 constitutes the valve device A of the main piston 28, and the pin 37 and the annular member 38 function as the main valve opening means of the valve device A. The valve device A has a shutoff valve action whose forward direction is from the pressureless chamber 44 formed on the circumferential side of the main piston 28 to the main pressure chamber 46 , and prevents the main piston 28 from moving to the left. As a result, the end surface of the valve rubber 33 is seated on the valve seat 43. In addition, this valve device A is arranged so that when the main pressure chamber 46 becomes negative pressure with respect to the no-pressure chamber 44 , that is, when the brake is released, the hydraulic fluid reservoir 47
The hydraulic fluid supplied into the pressureless chamber 44 through the replenishment hole 48 is supplied to the main pressure chamber 4 through the liquid path B of the piston 28, which consists of the hole 36, the axial passage hole 35, and the hole 45.
6. Main piston 2 equipped with such a valve device A
At the front end of the piston 8, a bolt 49 integrally provided with the spring receiver 41 extends toward the slave piston 29 side. A substantially hat-shaped spring receiver 50 is engaged with the head of this bolt 49. A return spring 52 is stretched in a compressed state between the spring receiver 50 and a spring receiver 51 formed on the main piston 28, and presses the spring receiver 50 against the rear end of the slave piston 29. The head of the bolt 49 is slidably fitted into an axial hole 29a formed at the rear end of the slave piston 29. The axial hole 29a is a through hole 53 of the slave piston 29 extending in a direction intersecting the hole 29a.
The bolt 49 is connected in the axial hole 29a.
The liquid pressed by the piston 29 is released to the outside of the slave piston 29. On the other hand, cup seals 56 and 57 are attached to the rear end large diameter section 54 and the front end large diameter section 55 of the slave piston 29, so that the front end large diameter section 54 and the rear end large diameter section 55
A pressureless chamber 58 is formed between the piston 29 and the bottom wall of the cylinder body 22, and a subordinate pressure chamber 59 is formed between the piston 29 and the bottom wall of the cylinder body 22. The slave piston 29 has a return spring 61 stretched between a spring receiving portion 60 formed at its front end and the bottom wall of the cylinder body 22.
is biased to the right by. The left side portion of the slave piston 29 is formed as a cylindrical portion 62 . A hole 72 is formed in the peripheral wall of the cylindrical portion 62, thereby providing liquid communication between the inner hole of the cylindrical portion 62 and the slave pressure chamber 59. As shown in FIG.
A dish-shaped chrysanthemum washer 64 having a circular ring 3a on the circumference is fitted and fixed, and inside the cylindrical portion 62 into which the chrysanthemum washer 64 is fitted, there is a center hole having approximately the same diameter as the center hole 64a of the washer 64. A spring receiver 65 having a spring shape 65a is disposed. In this way, the inner hole of the cylindrical portion 62 also communicates with the slave pressure chamber 59 via the center hole 65a of the spring receiver 65 and the center hole 64a of the chrysanthemum washer 64. A spring 68 is stretched between the spring receiver 65 and a valve rubber 67 attached to the tip of the valve rod 66, and urges the valve rod 66 to the right in the figure. The valve rod 66 is slidably fitted into an axial through hole 69 formed in the slave piston 29. A hydraulic fluid storage chamber 6 is provided at the rear end of the axial through hole 69.
9a is formed, and the chamber 69a communicates with the pressureless chamber 58 via a passage (not shown). The end of a bolt 70 inserted through the side wall of the cylinder body 22 is arranged so as to face inside the hydraulic fluid storage chamber 69a. As will be described later, when the operator releases the brake force and the slave piston 29 slides to the right, the bolt 70 catches one end of the valve rod 66 as shown and restricts further movement. When the slave piston 29 slides leftward, it releases contact with the valve rod 66 and functions as a valve opening member 70. Also, valve rod 66
is in contact with the bolt 70 in the non-operating state due to the biasing force of the spring 68. On the other hand, the bottom wall portion of the cylindrical portion 62 functions as a valve seat 71, and normally faces the end surface of the valve rubber 67 at a predetermined interval as shown. Such valve rubber 67, valve seat 71, valve rod 6
6. The spring 68 constitutes the valve device C of the slave piston 29, and the bolt 70 functions as the slave opening valve means 70 of the valve device C as described above. The valve device C has a shutoff valve action whose forward direction is from the pressureless chamber 58 formed on the circumferential side of the slave piston 29 to the slave pressure chamber 59 , and as the slave piston 29 moves to the left, The end face of the valve rubber 67 is seated on the valve seat 72. Also,
When the pressure in the slave pressure chamber 59 becomes negative with respect to the no-pressure chamber 58 , that is, when the brake is released, the valve device C is operated from the hydraulic fluid reservoir 47 through the supply hole 73, the no-pressure chamber 58, and the no-pressure chamber 58. A passage (not shown) communicating with the hydraulic fluid storage chamber 69a , the hydraulic fluid storage chamber 69a , the axial through hole 69, the hole 72 of the cylindrical member 62 or the spring receiver 65, and the center hole 65a, 64 of the chrysanthemum washer 64.
It is supplied to the slave pressure chamber 59 via the liquid path D consisting of a. In this way, in the brake master cylinder 1, the main pressure chamber 46 and the subordinate pressure chamber are controlled according to the movements of the main piston 28 and the subordinate piston 29 by the two valve devices A and C provided in the respective fluid passages B and D. 59 to the hydraulic fluid reservoir 47 can be restricted. An output port 74 is formed at the front end of the cylinder body 22 in communication with the slave pressure chamber 59 , and is connected to one wheel brake device via a pipe (not shown). Further, although not shown, an output port is formed in the intermediate portion of the cylinder body 22 so as to communicate with the main pressure chamber 46 , and this is connected to the other wheel brake device. A boss 82 is formed on the upper wall of the front end portion of the cylinder body 22, and the liquid connection hole 75 is connected to the supply hole 73.
It is constantly in communication with the pressureless chamber 58 via. Also,
A connecting cylindrical portion 77 of the hydraulic fluid reservoir 47 is press-fitted into the connecting hole 75 via a grommet seal 76 . The hydraulic fluid reservoir 47 has a known structure, and the upper opening of a cylindrical main body 78 is covered by a cap 79, and the inside is divided into first and second reservoir chambers 80 ,
81. A boss part 83 higher than the above-mentioned boss part 82 is formed on the upper wall of the middle part of the cylinder body 22, and the liquid connection port 84 is connected to the pressureless chamber 4 through the supply hole 48.
I am in constant communication with. In addition, a reservoir 47 is connected to the upper part of the liquid connection hole 84 via a grommet seal 85.
The other connecting cylinder portion 86 is press-fitted. The embodiment of the present invention is configured as described above,
Next, this action, effect, etc. will be explained. When the brake is not activated, each part is in the state shown. When the driver depresses a brake pedal (not shown) in this state, the main piston 28 is pressed via the output shaft of a negative pressure booster (not shown), and the pressure is transmitted to the slave piston 29 via the return spring 52. Thereby, the main piston 28 and the slave piston 2
9 move together. When the main piston 28 and the slave piston 29 move forward a predetermined distance to the left, the tips of the valve rods 32 and 66 separate from the pin 37 and the bolt 70, respectively, and the spring 3
The end surfaces of the valve rubbers 33, 67 come into contact with the valve seats 43, 71 due to the urging forces of the valve rubbers 4, 68. That is, valve devices A and C are closed. Therefore, the liquid paths B and D are blocked, and the hydraulic fluid reservoir 47 and the main pressure chamber 46
between the hydraulic fluid reservoir 47 and the slave pressure chamber 59
The pressure chambers 46 and 59 are placed in a non-communicating state, and the pressures in the main pressure chamber 46 and the subordinate pressure chamber 59 are increased. Thereafter, as the master and slave pistons 28 and 29 move forward, the hydraulic pressures in the main pressure chamber 46 and the slave pressure chamber 59 further increase. In this way, the desired brake can be applied to the vehicle. When the driver returns the brake pedal to release the brake force, the main piston 28 and the slave piston 29 move to the right under the hydraulic pressure of both pressure chambers 46 and 59 and the spring force of the pair of return springs 52 and 61. Return to . Then, the tips of the valve rods 32, 66 come into contact with the pin 37 and bolt 70, and the valve devices A, C
opens. As a result, the pressure liquid in the pressureless chamber 44 flows into the main pressure chamber 46 via the liquid path B, while the pressure liquid in the pressureless chamber 58 flows into the secondary pressure chamber 5 through the liquid path D.
9 and both pressure chambers 46 and 59 are replenished with the liquid. Thereafter, the above operation is repeated by pressing and releasing the brake pedal. Incidentally, the vacuum bleeding of the master cylinder 1 that performs the above-mentioned operation is performed by removing the cap 79 of the hydraulic fluid reservoir 47 and drawing suction from its opening with a vacuum pump. Air within the main pressure chamber 46 is sucked through passages that will later constitute liquid passages B and D. As the suction progresses, the pressure in the main pressure chamber 46 and the secondary pressure chamber 59 becomes smaller compared to atmospheric pressure, but as the pressure difference with atmospheric pressure increases, the guide member 24, which has a large pressure-receiving area, is forced to move toward the left. The force acts. If the wave washer 88 were not interposed, the guide member 24 would advance and the support member 39 would be advanced through the guide member 24. When the support member 39 moves forward, the force acts on the annular member 38, causing the annular member 38 to move forward, and at the same time pushing the pin 37 fitted into the annular member 38.
moves forward. When pin 37 moves forward, pin 3
7 comes into contact with the side wall of the hole 36 and presses the main piston 28 to the left. This causes the main piston 28 to move forward. In this state, the tip of the valve rod 32 remains in contact with the pin 37, so the valve device A is open. On the other hand, when the main piston 28 moves forward, the slave piston 29 is pressed via the return spring 52, and the slave piston 29 moves forward by this force. When the slave piston 29 moves forward by a predetermined distance, the tip of the valve rod 66 separates from the bolt 70, and the end surface of the valve rubber 67 seats on the valve seat 71. That is, the valve device C is closed. When the valve device C is closed in this way, the liquid path D is blocked, so that air in the secondary pressure chamber 59 cannot be vented from now on. However, in this embodiment, since the wave washer 88 is interposed in the gap in front of the guide member 24, the force that causes the guide member 24 to move forward due to the differential pressure is absorbed by the elastic force of the wave washer 88.
The guide member 24 does not move. In this manner, in this embodiment, since the guide member 24 does not move during vacuum bleeding, the support member 39 and the annular member 38 also remain in their original positions, and the main piston 28 and the slave piston 29 do not move either. Therefore, like the valve device A, the valve device C is maintained in the open state, and the air is promptly vented on the slave pressure chamber 59 side without being interrupted. Although the embodiments of the present invention have been described above, the present invention is of course not limited thereto, and various modifications can be made based on the technical idea of the present invention. For example, in the above embodiment, a wave washer 88 is used as the elastic member, and the spring force of the wave washer 88 prevents movement of the guide member 24, but instead of the wave washer 88, as shown in FIG. The seal ring 89 may be compressed in the axial direction and interposed therebetween. In that case, the seal ring 89 is
4, the seal ring 89 is attached to the circumferential surface of the annular stepped portion of
is placed between the vertical wall of the large diameter hole 23a of the cylinder hole 23 and the vertical wall of the guide member 24 so as to be compressed. Further, the support member 39 is press-fitted into the recessed end face of the guide member 24 to which the cup seal 27 is attached. In the case of the master cylinder 90 in which the seal ring 89 and the support member 39 are attached in this manner, the main piston 28 is
A pin 37 inserted into the hole 36 is placed so as to come into contact with one side wall of the hole 36, and the tip of the valve rod 32 is placed so as to come into contact with the pin 37. Even such an elastic member 89 has the same function and effect as a spring member as the wave washer 88, and its explanation will be omitted. [Effects of the Invention] As described above, according to the master cylinder of the present invention, the guide member does not move because the spring force of the spring member urges the guide member to a predetermined position.
Therefore, the end face of the valve rubber and the valve seat do not come into contact with each other during vacuum bleeding, so that air can be reliably bleed without extra effort.

[Brief explanation of drawings]

Fig. 1 is a side sectional view of a master cylinder according to an embodiment of the present invention, Fig. 2 is a perspective view of only the annular member and pin shown in Fig. A plan view of the washer, FIG. 4 is an enlarged sectional view of FIG. FIG. 8 is a sectional side view of a master cylinder according to another embodiment, FIG. 8 is a sectional view showing essential parts of a conventional master cylinder, and FIG. 9 is a perspective view showing only the circlip and transmission member of FIG. 8. In the figure, 22... cylinder body, 23
... Cylinder hole, 24 ... Guide member, 28 ... Main piston, 28a ... Shaft part, 29 ... Slave piston, 38 ... Annular member, 46 ... Main pressure chamber, 47
... Hydraulic fluid reservoir, 52, 61 ... Return spring,
59...Sensitive pressure chamber, 70...Valve opening member, 88...
Wave washer, 89... elastic member, A, C... valve device, B, D... liquid path.

Claims (1)

[Scope of utility model registration request] A cylinder body having a cylinder body formed with a cylinder hole, a shaft portion that is slidably inserted into the cylinder hole and extends to the outside of the cylinder body, and a head that has a larger diameter than the shaft portion and slides in the cylinder hole. a piston, a slave piston slidably inserted into the cylinder hole, a main pressure chamber formed between the main piston and the slave piston, and between the bottom of the cylinder hole and the slave piston. a secondary pressure chamber formed in the main pressure chamber and the secondary pressure chamber, respectively;
A fluid passage formed in each piston so as to communicate with the hydraulic fluid reservoir side, and a fluid passage provided in each of these fluid passages to limit communication between the main pressure chamber and the secondary pressure chamber and the hydraulic fluid reservoir, A valve member is provided at the end of the piston on the pressure chamber side and is biased from the pressure chamber side toward a valve seat provided on the piston by a valve spring, and the piston moves toward the pressure chamber side. a valve device configured to close the valve according to the opening of the cylinder hole; a guide member that guides sliding of the shaft portion of the main piston at the opening of the cylinder hole; a sealing member that seals between the shaft portion and the guide member; a retaining member provided at the opening of the cylinder hole to prevent the guide member from moving outside the cylinder hole; an annular member that is attached to the piston so as to be relatively movable within a predetermined range, that is fitted around the outer periphery of the shaft of the main piston and that can come into contact with the guide member, and that extends into a passage formed in the main piston;
A main body for opening the valve device of the main piston, the main piston having a shaft member that can be engaged with the valve member so as to displace the valve member from the valve seat when the annular member contacts the guide member. Valve opening means;
It is attached to the cylinder body, and engages with the valve member from the valve seat side until the slave piston moves a predetermined amount toward the slave pressure chamber side, and moves the valve member from the valve seat to the valve spring. In the master cylinder, the master cylinder is provided with a slave opening means for opening the valve device of the slave piston by disengaging the slave piston from its seat against an urging force, and the pressure acting on the inside of the cylinder hole of the guide member is applied to the outside thereof. The master cylinder is provided with a spring member that biases the guide member toward the retaining member by a biasing force that overcomes a differential pressure when the pressure becomes lower than the pressure acting on the master cylinder.