Master cylinder

Abstract

To provide a master cylinder that can easily accelerate a braking effect without requiring any special device and can shorten the time required for vacuum filling of brake fluid.SOLUTION: A plurality of openings on an inner peripheral surface of a cylinder body of a primary bypass port 8 are provided in a direction perpendicular to an advancing / retreating direction of a primary piston 4, and a plurality of openings on an inner peripheral surface of a cylinder body of a secondary bypass port 10 are provided in a direction perpendicular to an advancing / retreating direction of a secondary piston 5, or alternatively, an opening on the inner peripheral surface of the cylinder body of the primary bypass port 8 is formed in a shape of an elongated hole extending in a direction perpendicular to the advancing / retreating direction of the primary piston 4, and an opening on the inner peripheral surface of the cylinder body of the secondary bypass port 10 is formed in the shape of an elongated hole extending in a direction perpendicular to the advancing / retreating direction of the secondary piston 5.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a master cylinder that constitutes a braking system of an automobile. [Background technology]

[0002] In general, in a hydraulic brake system for an automobile, when the brake pedal is depressed, a push rod attached to the arm of the brake pedal pushes in a piston in a master cylinder, and as the pushed-in piston moves forward, the piston cup blocks the opening on the inner surface of the cylinder body of the bypass port that communicates with the reservoir tank, generating hydraulic pressure within the cylinder, which then activates each brake system.

[0003] In such hydraulic brake systems, the sooner the bypass port opening on the inner circumferential surface of the cylinder body can be blocked, the sooner the brakes can be applied. Therefore, at first glance, making the bypass port smaller seems effective. However, during vehicle manufacturing, when filling the brake fluid, the brake system is evacuated and the air is removed before the brake fluid is filled. However, if the bypass port opening on the inner circumferential surface of the cylinder body is made small, the vacuum filling of the brake fluid takes a long time.

[0004] Patent Document 1 discloses a master cylinder device comprising: a master cylinder; a reservoir tank connected to the master cylinder; a piston inserted into a cylinder chamber formed in a cylinder body of the master cylinder and movable back and forth; a master port opening on the inner periphery of the cylinder chamber and communicating with the reservoir tank; a pair of cup seals disposed on the inner periphery of the cylinder chamber and sandwiching the master port in the axial direction between the inner periphery of the cylinder chamber and the outer periphery of the piston; and a relief port drilled in the piston and communicating with the master port, wherein as the piston moves forward in the cylinder chamber, the relief port is closed by the cup seal disposed forward in the direction of movement of the piston, thereby generating brake fluid pressure in the brake fluid stored in a hydraulic chamber partitioned by the cylinder chamber and the piston, and wherein the relief port is inclined from the front to the rear in the direction of movement of the piston from the outer periphery to the inner periphery of the piston.

[0005] Patent Document 1 aims to improve the durability of the cup seal by tilting the relief port drilled in the piston, but it does not speed up the braking effect or prevent the vacuum filling time from becoming longer. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2021-066322 Summary of the Invention [Problem to be solved by the invention]

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a master cylinder that can easily speed up braking without requiring any special device and can shorten the time required for vacuum filling of brake fluid. [Means for solving the problem]

[0008] The master cylinder of the first aspect of the present invention comprises a cylinder body (2) having a generally cylindrical shape with a bottom, connected to a rear brake system and a front brake system, a reservoir tank (3) containing brake fluid and connected to the cylinder body (2), a primary piston (4) inserted into the rear side of the cylinder body (2) and provided so as to be able to move forward and backward, a secondary piston (5) inserted into the front side of the cylinder body (2) and provided so as to be able to move forward and backward, and a piston (5) provided between a front portion of the primary piston (4) and a rear portion of the secondary piston (5). a primary return spring (6); a secondary return spring (7) provided between the tip end of the cylinder body (2) and the front part of the secondary piston (5); a primary bypass port (8) and a primary inlet port (9) provided on the rear side of the cylinder body (2) and communicating with the reservoir tank (3); a secondary bypass port (10) and a secondary inlet port (11) provided on the front side of the cylinder body (2) and communicating with the reservoir tank (3); a primary outlet (12) provided on the rear side of the cylinder body (2) and communicating with the rear brake system; a secondary outlet (13) provided on the front side of the cylinder body (2) and communicating with the front brake system; a primary front piston cup (14) and a primary rear piston cup (15) sealing the inner peripheral surface of the cylinder body (2) and the outer peripheral surface of the primary piston (4); a secondary front piston cup (16) and a secondary rear piston cup (17) sealing the inner peripheral surface of the cylinder body (2) and the outer peripheral surface of the secondary piston (5). a secondary rear piston cup (17), the primary bypass port (8) having a primary opening (20) opening to the inner circumferential surface of the cylinder body (2) and a primary communication passage (21) communicating with the reservoir tank (3), a plurality of the primary openings (20) being provided in a direction perpendicular to the advancing and retreating direction of the primary piston (4), the secondary bypass port (10) having a secondary opening (22) opening to the inner circumferential surface of the cylinder body (2) and a secondary communication passage (23) communicating with the reservoir tank (3),The secondary opening (22) is provided in a plurality of pieces in a direction perpendicular to the direction of advancement and retreat of the secondary piston (5).

[0009] A master cylinder according to a second aspect of the present invention includes a cylinder body (2) having a generally cylindrical shape with a bottom, which is connected to a rear brake system and a front brake system; a reservoir tank (3) for storing brake fluid and connected to the cylinder body (2); a primary piston (4) inserted into the rear side of the cylinder body (2) and provided so as to be able to move back and forth; a secondary piston (5) inserted into the front side of the cylinder body (2) and provided so as to be able to move back and forth; and a piston (6) provided between a front portion of the primary piston (4) and a rear portion of the secondary piston (5). a primary return spring (6), a secondary return spring (7) provided between the tip end of the cylinder body (2) and the front part of the secondary piston (5); a primary bypass port (8) and a primary inlet port (9) provided on the rear side of the cylinder body (2) and communicating with the reservoir tank (3); a secondary bypass port (10) and a secondary inlet port (11) provided on the front side of the cylinder body (2) and communicating with the reservoir tank (3); a primary outlet (12) provided on the rear side of the cylinder body (2) and communicating with the rear brake system; a secondary outlet (13) provided on the front side of the cylinder body (2) and communicating with the front brake system; a primary front piston cup (14) and a primary rear piston cup (15) sealing the inner peripheral surface of the cylinder body (2) and the outer peripheral surface of the primary piston (4); a secondary front piston cup (16) and a secondary rear piston cup (17) sealing the inner peripheral surface of the cylinder body (2) and the outer peripheral surface of the secondary piston (5). a primary bypass port (8) having a primary opening (20) that opens into the inner peripheral surface of the cylinder body (2) and a primary communication passage (21) that communicates with the reservoir tank (3), the primary opening (20) being formed in the shape of an elongated hole that extends in a direction perpendicular to the advancing and retreating direction of the primary piston (4), the secondary bypass port (10) having a secondary opening (22) that opens into the inner peripheral surface of the cylinder body (2) and a secondary communication passage (23) that communicates with the reservoir tank (3),The secondary opening (22) is formed in the shape of an elongated hole extending in a direction perpendicular to the direction of advancement and retreat of the secondary piston (5). [Effects of the Invention]

[0010] According to the first aspect of the present invention, since a plurality of openings of the bypass port on the inner circumferential surface of the cylinder body are provided in a direction perpendicular to the direction of the piston's advance and retreat, even if the openings of the inner circumferential surface of the cylinder body of each bypass port are made small so that the piston can close the openings of the bypass port on the inner circumferential surface of the cylinder body in a short time in order to speed up braking, the total area of ​​the openings of the inner circumferential surface of the cylinder body of the plurality of bypass ports does not become small, and it is possible to shorten the vacuum filling time.

[0011] According to the second aspect of the present invention, the opening of the bypass port on the inner circumferential surface of the cylinder body is formed in the shape of an elongated hole that extends in a direction perpendicular to the direction of movement of the piston. Therefore, even if the length of the opening of the bypass port on the inner circumferential surface of the cylinder body in the direction of movement of the piston is reduced so that the piston can close the opening of the bypass port on the inner circumferential surface of the cylinder body in a short time in order to speed up braking, the area of ​​the opening of the bypass port on the inner circumferential surface of the cylinder body does not become smaller, and it is possible to shorten the vacuum filling time. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a side cross-sectional view showing an example of a master cylinder of the present invention. [Figure 2] FIG. 2 is a schematic enlarged view showing an example of an opening of a bypass port on an inner peripheral surface of a cylinder body according to the first embodiment of the present invention. [Figure 3] FIG. 5 is a schematic enlarged view showing an example of an opening of a bypass port on an inner peripheral surface of a cylinder body according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0014] FIG. 1 is a side cross-sectional view showing an example of a master cylinder 1 of the present invention, which constitutes a hydraulic brake system of an automobile.

[0015] 1, the master cylinder 1 includes a substantially cylindrical cylinder body 2 with a bottom. The cylinder body 2 is connected to the rear brake system and the front brake system, that is, the wheel cylinders and the like.

[0016] A primary outlet 12 is provided on the rear side of the cylinder body 2, and a primary outlet 13 communicates with the rear brake system. Further, a secondary outlet 13 is provided on the front side of the cylinder body 2, and a secondary outlet 14 communicates with the front brake system.

[0017] In the present invention, "forward" and "front" refer to the left side in Fig. 1 and the direction toward the bottom of the generally cylindrical cylinder body 2, i.e., the bottom wall. In the present invention, "rear" and "rear" refer to the right side in Fig. 1 and the direction toward the opening of the generally cylindrical cylinder body 2.

[0018] The master cylinder 1 is provided with a reservoir tank 3 for storing brake fluid, which is connected to the cylinder body 2.

[0019] A primary piston 4, which is provided so as to be able to move back and forth, is inserted into the rear side of the cylinder body 2, and a secondary piston 5, which is provided so as to be able to move back and forth, is inserted into the front side of the cylinder body 2. That is, the internal cylindrical portion of the cylinder body 2 is provided with two pistons 4, 5, which move back and forth while sliding along the inner circumferential surface thereof.

[0020] A primary return spring 6 is provided between the front portion of the primary piston 4 and the rear portion of the secondary piston 5, and a secondary return spring 7 is provided between the tip end inside the cylinder body 2 and the front portion of the secondary piston 5. The inner tip portion of the cylinder body 2 refers to the inner portion of the front end of the cylinder body 2, that is, the inner bottom wall portion of the cylinder body 2.

[0021] The primary piston 4 is provided with a primary front piston cup 14 and a primary rear piston cup 15 that seal the inner circumferential surface of the cylinder body 2 and the outer circumferential surface of the primary piston 4, and the secondary piston 5 is provided with a secondary front piston cup 16 and a secondary rear piston cup 17 that seal the inner circumferential surface of the cylinder body 2 and the outer circumferential surface of the secondary piston 5.

[0022] With the above configuration, as shown in FIG. 1, a primary hydraulic chamber 18 is defined by the inner circumferential surface of the cylinder body 2, the primary front piston cup 14, and the secondary rear piston cup 17, and a secondary hydraulic chamber 19 is defined by the inner circumferential surface of the cylinder body 2, the inner tip end of the cylinder body 2, and the secondary front piston cup 16.

[0023] A primary bypass port 8 and a primary inlet port 9 that communicate with the reservoir tank 3 are provided on the rear side of the cylinder body 2, and a secondary bypass port 10 and a secondary inlet port 11 that communicate with the reservoir tank 3 are provided on the front side of the cylinder body 2.

[0024] The primary bypass port 8, the primary inlet port 9, the secondary bypass port 10, and the secondary inlet port 11 are portions that communicate between the inside of the cylinder body 2 and the reservoir tank 3, and specifically, each includes a hole that opens into the inner circumferential surface of the cylinder body 2 and a communication passage that leads to the reservoir tank 3. In the present invention, the hole that opens into the inner circumferential surface of the primary bypass port 8 in the cylinder body 2 is referred to as a primary opening 20, the communication passage that leads to the reservoir tank 3 is referred to as a primary communication passage 21, the hole that opens into the inner circumferential surface of the secondary bypass port 10 in the cylinder body 2 is referred to as a secondary opening 22, and the communication passage that leads to the reservoir tank 3 is referred to as a secondary communication passage 23. The primary bypass port 8 and the secondary bypass port 10 have the function of returning brake fluid from the cylinder body 2 side to the reservoir tank 3, and the primary inlet port 9 and the secondary inlet port 11 have the function of supplying brake fluid from the reservoir tank 3 to the cylinder body 2 side.

[0025] In the master cylinder 1 of the first embodiment of the present invention, as shown in Fig. 2 (the arrows in Fig. 2 indicate the advancing and retreating directions of the pistons 4, 5), a plurality of primary openings 20 are provided in a direction perpendicular to the advancing and retreating direction of the primary piston 4, and a plurality of secondary openings 22 are provided in a direction perpendicular to the advancing and retreating direction of the secondary piston 5. That is, while the primary piston 4 and the secondary piston 5 advance to the left and retreat to the right in Fig. 1, a plurality of primary openings 20 and a plurality of secondary openings 22 are provided in a direction perpendicular to the advancing and retreating direction. In addition, when multiple primary openings 20 and multiple secondary openings 22 are provided, multiple primary communication passages 21 and multiple secondary communication passages 23 can be provided, i.e., the same number as the number of primary openings 20 and secondary openings 22, or even when multiple primary openings 20 and multiple secondary openings 22 are provided, it is also possible to provide one primary communication passage 21 and one secondary communication passage 23, so that each primary opening 20 shares one primary communication passage 21 and each secondary opening 22 shares one secondary communication passage 23.

[0026] Bypass ports are also called return ports, relief ports, compensation ports, etc., and are usually provided as one primary bypass port and one secondary bypass port, but in the first aspect of the present invention, multiple ports of each type are provided in the above directions.

[0027] In the master cylinder 1, the bypass ports 8, 10, specifically the primary opening 20 and secondary opening 22 of the bypass ports 8, 10, are blocked by the primary piston 4 and secondary piston 5, specifically the primary front piston cup 14 and secondary front piston cup 16, respectively, thereby increasing the hydraulic pressure in the primary hydraulic chamber 18 and secondary hydraulic chamber 19 and activating each brake system. Therefore, if the primary opening 20 and the secondary opening 22 are formed as small as possible, the time until the primary opening 20 and the secondary opening 22 are closed can be shortened, and the brakes can be applied sooner. However, as mentioned above, the smaller the primary opening 20 and the secondary opening 22 are made, the longer it takes to vacuum fill the brake fluid during vehicle manufacture. Therefore, in the first aspect of the present invention, a plurality of primary openings 20 and a plurality of secondary openings 22 are provided in a direction perpendicular to the advancing and retracting directions of the pistons 4 and 5. Specifically, for example, it is desirable to provide two to three primary openings 20 and two to three secondary openings 22.

[0028] If multiple primary openings 20 and secondary openings 22 are provided in the same direction as the advance / retraction direction of the pistons 4 and 5, the advance distance of the pistons 4 and 5 to close all of the multiple primary openings 20 and secondary openings 22 will be longer, resulting in slower brake operation. In contrast, even if multiple primary openings 20 and multiple secondary openings 22 are provided in a direction perpendicular to the direction of advance and retreat of the pistons 4 and 5, as in the present invention, the advance distance of the pistons 4 and 5 to close all of the multiple primary openings 20 and secondary openings 22 is the same as when only one primary opening 20 and one secondary opening 22 are provided, so brake operation does not become slower.

[0029] In the first aspect of the present invention, it is preferable to make the primary openings 20 and the secondary openings 22 as small as possible. Specifically, for example, when existing primary bypass ports and secondary bypass ports are provided, it is preferable to make the additionally provided multiple primary openings 20 and secondary openings 22 the same size as or smaller than the cylinder body inner peripheral surface openings of the existing primary bypass ports and secondary bypass ports. Even if multiple primary openings 20 and secondary openings 22 are provided in this way, the travel distance of the pistons 4, 5 to close them does not change, and therefore brake operation does not become slower.

[0030] Making the primary opening 20 and the secondary opening 22 as small as possible compared to the opening of a normal bypass port on the inner circumferential surface of the cylinder body is desirable because it shortens the travel distance of the pistons 4, 5 to close the primary opening 20 and the secondary opening 22, thereby enabling earlier brake activation. Here, "as small as possible compared to the opening of a normal bypass port on the inner circumferential surface of the cylinder body" means, for example, that the diameters of the primary opening 20 and the secondary opening 22 are each less than 1 mm.

[0031] As described above, in the first aspect of the present invention, a plurality of primary openings 20 and a plurality of secondary openings 22 are provided, and therefore, in a preferred aspect, even if the primary openings 20 and secondary openings 22 are made small, the total area of ​​the plurality of primary openings 20 and secondary openings 22 is relatively large, making it possible to shorten the time required for vacuum filling of brake fluid during vehicle manufacture.

[0032] In the master cylinder 1 of the second embodiment of the present invention, as shown in Fig. 3 (the arrows in Fig. 3 indicate the advancing and retreating directions of the pistons 4, 5), the primary opening 20 is formed in the shape of an elongated hole extending in a direction perpendicular to the advancing and retreating direction of the primary piston 4, and the secondary opening 22 is formed in the shape of an elongated hole extending in a direction perpendicular to the advancing and retreating direction of the secondary piston 5. Note that the openings of the normal primary bypass port and secondary bypass port on the inner peripheral surface of the cylinder body are formed in the shape of a perfect circle.

[0033] 3 showing an example of the second aspect of the present invention, one primary bypass port 8 and one secondary bypass port 10 are provided, and these primary openings 20 and secondary openings 22 are provided in the shape of holes that are long in a direction perpendicular to the advancing and retreating direction of the pistons 4 and 5. On the other hand, the lengths of the primary openings 20 and secondary openings 22 in the same direction as the advancing and retreating direction of the pistons 4 and 5 are set to be the same as or preferably shorter than the diameters of the openings on the inner peripheral surface of the cylinder body of normal primary bypass ports and secondary bypass ports, for example, less than 1 mm.

[0034] As described above, by providing the second aspect of the present invention, similar to the first aspect, the travel distance of the pistons 4, 5 for closing the primary opening 20 and the secondary opening 22 can be shortened, thereby speeding up brake operation, and since the sizes of the primary opening 20 and the secondary opening 22 can be made relatively large, it is possible to shorten the time required for vacuum filling of brake fluid during vehicle manufacture. In the second embodiment of the present invention, similarly to the first embodiment, it is also possible to provide a plurality of primary openings 20 and secondary openings 22 in a direction perpendicular to the advancing and retracting directions of the pistons 4 and 5, respectively.

[0035] Next, the operation of the master cylinder 1 of the present invention will be described.

[0036] When a brake pedal (not shown) is depressed, a push rod (not shown) attached to the arm of the brake pedal pushes forward a primary piston 4 of the master cylinder 1, thereby generating hydraulic pressure in a primary hydraulic chamber 18, which in turn advances a secondary piston 5. As the primary piston 4 and secondary piston 5 advance, a primary front piston cup 14 and a secondary front piston cup 16 close the cylinder body inner circumferential surface openings (primary opening 20 and secondary opening 22) of the primary bypass port 8 and the secondary bypass port 10, respectively. This increases the hydraulic pressure in the primary hydraulic chamber 18 and the secondary hydraulic chamber 19, and the increased hydraulic pressure passes through a primary delivery port 12 and a secondary delivery port 13 to activate a rear brake system and a front brake system (not shown), respectively. When the brake pedal is released, the primary piston 4 and the secondary piston 5 are returned to their initial positions by the biasing forces of the primary return spring 6 and the secondary return spring 7, respectively.

[0037] In the present invention, the primary opening 20 and the secondary opening 22 are small and provided in the same direction as the advancement and retreat of the pistons 4, 5. Therefore, the pistons 4, 5 can move a short distance to close the primary opening 20 and the secondary opening 22 of the primary front piston cup 14 and the secondary front piston cup 16 by the advancement of the primary piston 4 and the secondary piston 5, thereby enabling the brake to be activated earlier. [Explanation of symbols]

[0038] 1 master cylinder 2 Cylinder body 3 Reservoir tank 4 Primary Piston 5 Secondary Piston 6 Primary return spring 7 Secondary return spring 8 Primary Bypass Ports 9 Primary Inlet Port 10 Secondary Bypass Port 11 Secondary Inlet Port 12 Primary outlet 13 Secondary outlet 14 Primary front piston cup 15 Primary rear piston cup 16 Secondary front piston cup 17 Secondary rear piston cup 18 Primary hydraulic chamber 19 Secondary hydraulic chamber 20 Primary Opening 21 Primary connecting passage 22 Secondary opening 23 Secondary communication passage

Claims

1. a substantially cylindrical cylinder body (2) with a bottom connected to the rear brake system and the front brake system; a reservoir tank (3) that contains brake fluid and is connected to the cylinder body (2); a primary piston (4) inserted into the rear side of the cylinder body (2) and provided so as to be able to move back and forth; a secondary piston (5) inserted into the front side of the cylinder body (2) and provided so as to be able to move forward and backward; a primary return spring (6) provided between a front portion of the primary piston (4) and a rear portion of the secondary piston (5); a secondary return spring (7) provided between the inner tip of the cylinder body (2) and a front part of the secondary piston (5); a primary bypass port (8) and a primary inlet port (9) provided on the rear side of the cylinder body (2) and communicating with the reservoir tank (3); a secondary bypass port (10) and a secondary inlet port (11) provided on the front side of the cylinder body (2) and communicating with the reservoir tank (3); a primary outlet (12) provided on the rear side of the cylinder body (2) and communicating with the rear brake system; a secondary outlet (13) provided on the front side of the cylinder body (2) and communicating with the front brake system; a primary front piston cup (14) and a primary rear piston cup (15) for sealing an inner peripheral surface of the cylinder body (2) and an outer peripheral surface of the primary piston (4); a secondary front piston cup (16) and a secondary rear piston cup (17) that seal an inner peripheral surface of the cylinder body (2) and an outer peripheral surface of the secondary piston (5); the primary bypass port (8) includes a primary opening (20) that opens to the inner circumferential surface of the cylinder body (2) and a primary communication passage (21) that communicates with the reservoir tank (3), and a plurality of the primary openings (20) are provided in a direction perpendicular to the advancing and retreating direction of the primary piston (4); a secondary bypass port (10) having a secondary opening (22) that opens to the inner peripheral surface of the cylinder body (2) and a secondary communication passage (23) that communicates with the reservoir tank (3), and a plurality of secondary openings (22) are provided in a direction perpendicular to the direction of advance and retreat of the secondary piston (5).

2. a substantially cylindrical cylinder body (2) with a bottom connected to the rear brake system and the front brake system; a reservoir tank (3) that contains brake fluid and is connected to the cylinder body (2); a primary piston (4) inserted into the rear side of the cylinder body (2) and provided so as to be able to move back and forth; a secondary piston (5) inserted into the front side of the cylinder body (2) and provided so as to be able to move forward and backward; a primary return spring (6) provided between a front portion of the primary piston (4) and a rear portion of the secondary piston (5); a secondary return spring (7) provided between the inner tip of the cylinder body (2) and a front part of the secondary piston (5); a primary bypass port (8) and a primary inlet port (9) provided on the rear side of the cylinder body (2) and communicating with the reservoir tank (3); a secondary bypass port (10) and a secondary inlet port (11) provided on the front side of the cylinder body (2) and communicating with the reservoir tank (3); a primary outlet (12) provided on the rear side of the cylinder body (2) and communicating with the rear brake system; a secondary outlet (13) provided on the front side of the cylinder body (2) and communicating with the front brake system; a primary front piston cup (14) and a primary rear piston cup (15) for sealing an inner peripheral surface of the cylinder body (2) and an outer peripheral surface of the primary piston (4); a secondary front piston cup (16) and a secondary rear piston cup (17) that seal an inner peripheral surface of the cylinder body (2) and an outer peripheral surface of the secondary piston (5); the primary bypass port (8) includes a primary opening (20) that opens to the inner circumferential surface of the cylinder body (2) and a primary communication passage (21) that communicates with the reservoir tank (3), the primary opening (20) being formed in the shape of an elongated hole that extends in a direction perpendicular to the advancing and retreating direction of the primary piston (4); a secondary bypass port (10) including a secondary opening (22) that opens to the inner peripheral surface of the cylinder body (2) and a secondary communication passage (23) that communicates with the reservoir tank (3), and the secondary opening (22) is provided in the shape of an elongated hole that extends in a direction perpendicular to the direction of advancement and retreat of the secondary piston (5).

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