Valve unit for anti-lock braking system

Abstract

The valve unit for an antilock brake system includes a valve body (10, 110), a cutoff piston (18, 118) movable within an internal cavity (22) of the valve body, and a sliding contact seal disposed at the interface between the cutoff piston and the internal cavity. The internal cavity is hydraulically connectable with an outlet port (14) to a brake caliper, an inlet line (20), and an expansion chamber (33). The seal defines, together with the cutoff piston and the internal cavity, an axially movable hydraulic chamber (15, 115). The cutoff piston is movable between a first position, in which the hydraulic chamber is in fluid communication with the expansion chamber and the outlet port but not with the inlet line, and a second position, in which the hydraulic chamber is in fluid communication with the outlet port and the inlet line but not with the expansion chamber.

Description

Technical Field

[0001] The present invention relates to a valve unit for a hydraulic brake system for controlling an anti-lock function of wheels of a vehicle, particularly a two-wheeled vehicle such as a bicycle.

Background Art

[0002] An anti-lock brake system (hereinafter referred to as "ABS") is mounted on a vehicle equipped with a hydraulic brake in order to reduce the impact of sudden stops and prevent skidding, that is, uncontrollable sliding. In an ABS brake system, each wheel of the vehicle is equipped with a brake disk and an associated phonic wheel sensor or equivalent element that is rotatably integrated with the brake disk. The sensor detects the rotational speed of the associated wheel and transmits a signal indicating the rotational speed to an electronic control unit (ECU) that processes the received rotational signal. A brake caliper is attached to each brake disk. A master cylinder actuated by a control device (a handle lever in the case of a bicycle) actuates the brake caliper via each hydraulic circuit, and an ABS valve unit is attached to each of the brake calipers. Each ABS valve unit controls the flow rate and pressure of the brake fluid towards the associated brake caliper in response to an electrical control signal from the electronic control unit. When the ECU detects a state indicating that wheel lock is imminent, it actuates each ABS valve to lower the brake fluid pressure of the affected wheel and reduce the braking force of this wheel. As a result, the wheel continues to be braked but is able to rotate. This process is repeated continuously several times per second during braking to prevent the vehicle from skidding.

[0003] A valve unit for a vehicle anti-lock braking system has been proposed, comprising a valve body having an outlet port hydraulically connectable to a brake caliper, an inlet port hydraulically connectable to a master cylinder, and a first and second hydraulic chambers fluidly communicating with the outlet port. The outlet passage provides fluid communication between the first hydraulic chamber and the expansion chamber, and the bypass passage provides fluid communication between the inlet port and the outlet port. The piston is actuated by a solenoid controlled by an electronically controlled unit against the force of an elastic element and is axially movable within the hydraulic chamber. In a normal braking state, the force of the elastic element holds the piston in a position that blocks the outlet passage but not the bypass passage. In a locked-wheel braking state, the piston moves to a position that blocks the bypass passage, while the outlet passage is opened, so that some of the brake fluid is discharged from the main hydraulic chamber into the outlet chamber. As a result, the pressure on the caliper decreases and the wheel lock is released.

[0004] In a recently proposed solution, the movable piston in the first hydraulic chamber has a longitudinal cavity extending through the piston between the end face of the piston facing the outlet port and the transverse passage ending on the side of the piston. Overall, the piston has a first cross-section facing the outlet port and a second cross-section facing away from the outlet port on the opposite side of the first cross-section, with the first cross-section having a smaller area than the second cross-section.

[0005] The longitudinally extending cavity of the movable piston causes a pressure drop in the main hydraulic chamber, which simultaneously reduces the pressure of the brake fluid at the branch point of the hydraulic circuit extending from the outlet port to the brake caliper. This reduces the braking force exerted by the brake caliper, releasing the wheel lock. [Disclosure of the Invention]

[0006] The main objective of the present invention is to provide an improved ABS valve unit that can operate more quickly and effectively than known ABS valve units. In particular, it is desirable to provide an ABS valve unit having a higher operating frequency, characterized by a sequence of brake activation and release cycles with very short time intervals.

[0007] According to one embodiment, the present invention provides a valve unit for a hydraulic brake system that controls the anti-lock function of a vehicle's wheels, as defined in claim 1. Preferred embodiments are defined in the dependent claims.

[0008] In summary, a valve unit for a vehicle's antilock braking system comprises a valve body, a cutoff piston movable within an internal cavity of the valve body, and a sliding contact seal element positioned at the interface between the cutoff piston and the internal cavity. The valve body has an outlet port hydraulically connectable to a brake caliper, an inlet hydraulically connectable to a master cylinder, and an axially elongated internal cavity hydraulically connectable to the inlet and outlet ports. The valve body further comprises an expansion chamber expandable against an elastic element, an outflow passage between the cylindrical cavity and the expansion chamber, and a check valve positioned between the expansion chamber and the inlet. The cutoff piston is axially movable within the internal cavity against the action of the associated elastic element. The sliding contact seal element, together with the cutoff piston and the internal cavity, defines an axially movable hydraulic chamber. The cutoff piston is movable between two axial positions as follows: The piston is biased by an actuator such as a solenoid, compressing the associated elastic element and moving the hydraulic chamber to a first axial position, which is in fluid communication with the expansion chamber and outlet port, but not with the intake line, and A second axial position in which the piston is not biased by the solenoid actuator, the associated elastic elements are released, and the piston is positioned such that it fluidly communicates with the hydraulic chamber, the outlet port and the inlet conduit, but not with the expansion chamber. [Brief explanation of the drawing]

[0009] [Figure 1] This is a longitudinal cross-sectional view of a valve unit according to the first embodiment of the present invention in different operating states. [Figure 2] This is a longitudinal cross-sectional view of a valve unit according to the first embodiment of the present invention in different operating states. [Figure 3] This is a longitudinal cross-sectional view of a valve unit according to the first embodiment of the present invention in different operating states. [Figure 4] This is a longitudinal cross-sectional view of a valve unit according to the first embodiment of the present invention in different operating states. [Figure 5] This is a longitudinal cross-sectional view of a valve unit under different operating states according to a second embodiment of the present invention. [Figure 6] This is a longitudinal cross-sectional view of a valve unit under different operating states according to a second embodiment of the present invention. [Figure 7] This is a longitudinal cross-sectional view of a valve unit under different operating states according to a second embodiment of the present invention. [Figure 8] This is a longitudinal cross-sectional view of a valve unit under different operating states according to a second embodiment of the present invention. [Modes for carrying out the invention]

[0010] To ensure that the present invention is clearly understood, several preferred embodiments are now described illustratively with reference to the accompanying drawings. Referring first to Figures 1-4, reference numeral 10, as a whole, specifies an ABS valve unit for an antilock braking system for a vehicle, in particular a two-wheeled vehicle, typically a bicycle wheel.

[0011] The valve unit 10 has an elongated shape in the direction defined herein as longitudinal or axial, with respect to a longitudinal axis x. In this context, terms indicating position or direction, such as "longitudinal," "transverse," and "radial," should be constructed with reference to the x-axis.

[0012] The valve unit 10 includes a body 11 that defines the longitudinal direction of operation. In this example, the body 11 has an overall elongated shape in the longitudinal direction and has a first end 12 and a second end 13 opposite to the first end.

[0013] The main body 11 forms an outlet port 14 that can be hydraulically connected to a brake caliper (not shown) on the vehicle's wheels, and an inlet port 17 that can be hydraulically connected to a master cylinder (or main cylinder) (not shown) that is operationally associated with the vehicle's drive control device. The drive control device is typically a hand lever mounted on the handlebars of a bicycle.

[0014] The valve unit includes a shut-off valve that has the effect of blocking direct fluid communication between the inlet port and the outlet port, as described below.

[0015] The shut-off valve includes an axially extending cylindrical cavity 22 formed in the body 11, which is hydraulically connectable to an outlet port 14 and an inlet port 17. The cylindrical cavity 22 slidably receives a piston 18 in the axial direction.

[0016] The inlet pipe 20 hydraulically connects the inlet port 17 to the cylindrical cavity 22 and opens into the cylindrical cavity 22 via a bore 21.

[0017] The cutoff piston 18 is actuated axially within a cylindrical cavity 22 by a drive stem 29 connected in a well-known manner to an actuating solenoid (not shown), which is actuated by electrical control from an electronic control unit (not shown) that receives and processes electrical signals from each rotation sensor provided on the brake disc of each wheel. The structural and operational features of the electronic control unit are well-known and are not relevant to understanding the present invention, and therefore will not be described herein.

[0018] The elastic element 19, preferably a compression spring, disposed within the cylindrical cavity 22 elastically biases the cutoff piston 18 toward the end 13 of the body against the thrust action exerted by the drive stem 29.

[0019] A plurality of seals are provided at the interface between the cutoff piston and the cylindrical cavity 22 to effect a sliding contact sealing action between the cutoff piston 18 and the body 11 of the valve unit.

[0020] In the embodiment shown in FIGS. 1 - 4, the cutoff piston 18 has a laterally narrowed or reduced diameter intermediate portion 28 which, together with the cylindrical cavity 22, defines a hydraulically movable chamber 15 which has two axially opposed ends respectively bounded by first and second O - ring seals 23, 24 attached to the cutoff piston 18.

[0021] The third and fourth O - ring seals 25, 26 are attached to the cutoff piston 18 in positions axially outside and in proximity to the first and second seals 23, 24.

[0022] A second cylindrical cavity 34 is formed in the body 11 and communicates with the cylindrical cavity 22 via an outflow passage 31 which opens at one end of the second cylindrical cavity 34 disposed toward the inlet port 17.

[0023] The second cylindrical cavity 34 is in fluid communication with the inflow conduit 20 via a check valve 30 interposed between the hydraulically expandable chamber and a junction on the inflow conduit located intermediate the inlet port 17 and the cylindrical cavity 2C.

[0024] The second cylindrical cavity 34 houses a floating piston 27 and a second compression spring element 32 that biases the floating piston 27 toward the check valve and, consequently, toward the inlet port. Together, the second cylindrical cavity 34 and the floating piston 27 define a hydraulic expansion chamber 33 (Figure 2). As will be described later, when pressurized brake fluid is introduced into the expansion chamber 33 from the axially movable hydraulic chamber 15 through the outflow passage 31, the floating piston 27 moves toward the inlet port 17 and the check valve 30, against the biasing force of the second spring element 32.

[0025] Figure 1 shows the ABS valve unit in a normal brake operating state, for example, when the braked wheel is not locked. The ABS actuation solenoid is not operating. The compression spring 19 biases the cutoff piston 18 to the right, holding it so as to abut against the end of the right end 13 of the main body 11. The bore 21 of the inlet line communicates with an axially movable hydraulic chamber 15. Brake fluid entering from the inlet port passes through the ABS valve unit, through the inlet line 20 into the axially movable hydraulic chamber 15, and exits from the outlet port 14. The annular seal 23 is interposed between the axially movable hydraulic chamber 15 and the outlet passage 31, thereby preventing the brake fluid from reaching the empty expansion chamber 33. The floating piston 27 is biased by the spring 32 so as to abut against one end of the second cylindrical cavity on the check valve 30 side.

[0026] In a wheel brake lock state (Figure 2), the vehicle's electronic control unit receives a speed signal from the wheel sensor indicating that at least one of the vehicle's wheels is locked or skidding. Under these circumstances, the electronic control unit sends an actuation signal to the actuation solenoid of the ABS valve unit 10. The stem 29 of the solenoid moves the cutoff piston 18 to the left. Because the seal 24 is interposed between the bore 21 of the inlet pipe 20 and the hydraulic chamber 15, the bore 21 of the inlet pipe is no longer in communication with the axially movable hydraulic chamber 15.

[0027] Therefore, the hydraulic circuit from the master cylinder to the brake caliper is temporarily interrupted and divided into two temporarily disconnected parts by the cutoff piston 18. The hydraulic chamber 15 is now in communication with the brake caliper and the expansion chamber 33 (Figure 2), but is no longer in communication with the upstream inlet port and the master cylinder.

[0028] The high pressure of the brake fluid, which is contained in the circuit portion downstream of the cutoff piston and now communicates with the expansion chamber 33 via the hydraulic chamber 15 and the outflow passage 31, causes the floating piston 27 to move to the left against the elastic force of the spring 32. The volume of the expansion chamber 33 increases, and as a result, the pressure of the brake fluid located in the circuit portion between the ABS unit and the brake caliper drops instantaneously.

[0029] Subsequently, the brake caliper is released, allowing the brake disc and associated wheel to rotate. The solenoid is temporarily deactivated by the electronic control unit, thereby causing the spring 19 to bias the cutoff piston 18 to the right (Figure 3), returning it to its initial position where direct fluid communication from the inlet pipe 20 to the outlet port 14 via the axially movable hydraulic chamber 15 is resumed.

[0030] The solenoid immediately reactivates by moving the cutoff piston 18 to the left again, according to a known operating mode, closing the direct fluid communication between the brake caliper and the master cylinder, and restoring fluid communication between the brake caliper and the expansion chamber 33, into which even higher pressure brake fluid is injected, further increasing the volume of the expansion chamber. As the volume increases further, the fluid pressure in the brake caliper decreases instantaneously again and is released.

[0031] The above sequence continues for several seconds, with each movement causing the floating piston 27 to move further to the left and the volume of the expansion chamber 33 to gradually increase. When the braking action is finished, the user releases the brake control, which causes the brake fluid pressure in the inlet line 20 to drop to a value lower than the brake fluid pressure in the expansion chamber 33. This pressure difference opens the check valve 30 (Figure 4), which extends the spring 32, causing the floating piston 27 to be biased towards the inlet line 20, emptying the expansion chamber 33 and allowing the brake fluid accumulated in the expansion chamber to return to the brake circuit.

[0032] As can be understood, the operation of the ABS valve unit involves alternating short-length and rapid movements of a thin, low-mass cutoff piston, accompanied by a tight displacement over time of minute amounts of brake fluid rapidly discharged into the expansion chamber, thus enabling very rapid operation.

[0033] Figures 5 to 8 show a second embodiment of the ABS valve unit 10 according to the present invention. The general configuration of the valve unit depicted in Figures 5 to 8 is considered to be the same as that of the embodiments in Figures 1 to 4, except for the arrangement of the cutoff piston and associated seals. Accordingly, the following description will be made almost exclusively by referring to the differences between the second embodiment and the first embodiment, but it will be understood that any parts not described are considered to be the same or substantially the same as the parts already described with reference to Figures 1 to 4.

[0034] The cutoff valve of the second embodiment provides an axially extended cylindrical cavity 22 that is in fluid communication with the outlet port 14, the inlet port 17, and the expansion chamber 33. The cylindrical cavity 22 slidably receives the cutoff piston 118 in the axial direction.

[0035] The cutoff piston 118 has a laterally tapered or small-diameter intermediate section 128 on which a movable lip seal 123 is mounted, making sliding contact with the cylindrical cavity 22.

[0036] The cutoff piston 118 has at least one lateral passage 130a, 130b formed therein, extending into the interior of the piston and opening through at least two axially spaced openings 131, 132. The first opening 131 is located in a narrow portion of the piston close to the outflow passage 31, and the second opening 132 is located on the cylindrical surface 129 of the piston away from the outflow passage 31.

[0037] Preferably, as shown in the embodiments of Figures 5 to 8, a plurality of interconnected lateral passages 130a, 130b are provided, extending within the cutoff piston and opening onto the narrow intermediate section 128 and the lateral cylindrical surface 129 of the piston at one or more positions spaced axially apart from the narrow section 128.

[0038] The lateral passages 130a, 130b may include at least one passage having an inclined section 130a that opens onto the narrow portion 128 of the piston and merges in the central zone of the piston, and at least two radial sections 130b that merge with the inclined section 130a in the inner zone of the cutoff piston. Various arrangements other than those shown are possible. The multiple internal passages may be arranged in an axial plane spaced at an angle to the x-axis to provide multiple circumferentially spaced openings in the narrow portion of the piston and its cylindrical surface. The radial or inclined arrangement of the portions constituting the lateral passages, as well as their number and size, can be according to various arrangements, as long as fluid communication between the narrow portion of the piston and its cylindrical surface is achieved.

[0039] The static lip seal 124 is attached to the body 11 in the radially enlarged portion 122 of the cavity 22 and is positioned at an axial intermediate position between the outflow channel 31 and the inflow pipe 20.

[0040] The cutoff piston 118, together with the cylindrical cavity 22, defines an axially movable or axially extendable hydraulic chamber 115, which has two axially opposing ends surrounded by lip seals 123, 124 that act in sliding contact at the interface between the cutoff piston 118 and the cylindrical cavity 22.

[0041] The third and fourth O-ring seals 25 and 26 are mounted on the cutoff piston 118 and the body 11, respectively, in close proximity to the lip seals 123 and 124, and axially outward relative to them. Seal 125 makes sliding contact with the cylindrical cavity 22, and seal 126 makes sliding contact with the cylindrical surface 129 of the cutoff piston 118.

[0042] Figure 5 shows the ABS valve unit in a normal braking state, i.e., without locking the braked wheel. The ABS activation solenoid is not activated. The compression spring 19 holds the cutoff piston 118 biased to the right and abuts against the end bearing at the right end 13 of the main body 11. The bore 21 of the inlet pipe 20 communicates with the hydraulic chamber 115 via the passage 130. Brake fluid entering from the inlet port 17 passes through the ABS valve unit and enters the hydraulic chamber 115 from the inlet pipe 20 via passages 130b and 130a, and then exits from the outlet port 14. The movable lip seal 123 separates the hydraulic chamber 115 from the outlet passage 31, preventing brake fluid from reaching the empty expansion chamber 33. The floating piston 27 is biased by the spring 32 and abuts against one end of the second cylindrical cavity on the check valve 30 side.

[0043] In the wheel brake lock state (Figure 6), the vehicle's electronic control unit receives a speed signal from the wheel sensor indicating that at least one of the vehicle's wheels is locked or skidding. Under these conditions, the electronic control unit sends an actuation signal to the actuation solenoid of the ABS valve unit 110. The stem 29 of the solenoid moves the cutoff piston 118 toward the outflow passage side (left side, in the direction of arrow A). Because the lip seal 124 is interposed between the bore 21 and the passage 130b of the cutoff piston 118, the inflow pipe 20 is no longer in communication with the hydraulic chamber 115.

[0044] Therefore, the hydraulic circuit from the master cylinder to the brake caliper is temporarily interrupted by the cutoff piston 118, dividing it into two temporarily disconnected sections. The hydraulic chamber 115 is now in communication with the brake caliper and the expansion chamber 33 (Figure 6), but is no longer in communication with the inlet port and the master cylinder upstream of it.

[0045] The high pressure of the brake fluid, contained in the circuit portion downstream of the cutoff piston and now communicating with the expansion chamber 33 via the hydraulic chamber 115 and the outflow passage 31, causes the floating piston 27 to move to the left against the elastic force of the spring 32. The volume of the expansion chamber 33 increases, and as a result, the pressure of the brake fluid contained in the circuit portion between the ABS unit and the brake caliper drops instantaneously.

[0046] Subsequently, the brake caliper is released, allowing the brake disc and associated wheel to rotate. The solenoid is momentarily deactivated by the electronic control unit, thereby allowing the spring 19 to deflect the cutoff piston 118 to the right (Figure 7, arrow B) and return it to its initial position. Since the opening 132 of the flow path 130b has passed the lip seal 124, direct fluid communication is restored from the inlet pipe 20 to the outlet port 14 through the axially movable hydraulic chamber 115.

[0047] The solenoid is immediately reactivated by moving the cutoff piston 118 back to the left according to a known operating mode, which returns the opening 132 to the left of the lip seal 124, closing the direct fluid communication between the brake caliper and the master cylinder, and setting the brake caliper to fluid communication with the expansion chamber 33 again. In this way, an additional amount of high-pressure brake fluid is injected into this chamber, further increasing the volume of the expansion chamber. As the volume increases further, the fluid pressure inside the brake caliper decreases instantaneously and is released.

[0048] The above sequence continues for several seconds, with each movement causing the floating piston 27 to move further to the left and the volume of the expansion chamber 33 to gradually increase. When the braking action is finished, the user releases the brake control, which reduces the pressure of the brake fluid in the inlet pipe 20 to a value lower than the pressure of the brake fluid in the expansion chamber 33. This pressure difference opens the check valve 30 and releases the spring 32, allowing the floating piston 27 to be deflected towards the inlet pipe 20, emptying the expansion chamber 33 and returning the brake fluid that had accumulated in the expansion chamber back into the brake circuit.

[0049] In the embodiments shown in Figures 5 to 8, it will be understood that the operating stroke of the cutoff piston 118 is extremely short, which is necessary to move the opening 132 of the flow path 130b from one side to the other of the lip seal 124. Since the lip seal exhibits a sliding contact sealing action along a contact area with a very short axial length, the operating stroke of the cutoff piston can be made very short accordingly, enabling very rapid operation with a displacement close to the time at which the minimum volume of brake fluid is rapidly discharged into the expansion chamber.

[0050] While specific embodiments of the present invention have been described, it should be understood that this disclosure is provided for illustrative purposes only and should not limit the invention in any way. Various modifications will be apparent to those skilled in the art in consideration of the above-described embodiments. The scope of the present invention is limited only by the appended claims.

Claims

1. A valve unit for a vehicle's antilock braking system, - A valve body (11, 110) comprising an outlet port (14) that can be hydraulically connected to a brake caliper, an inlet conduit (20) that can be hydraulically connected to a master cylinder, an axially elongated internal cavity (22) that can be hydraulically connected to the inlet conduit (20) and the outlet port (14), an expansion chamber (33) that can expand against the action of an elastic element (32), an outflow passage (31) between the internal cavity (22) and the expansion chamber (33), and a check valve (30) positioned between the expansion chamber (33) and the inlet conduit (20), - Cutoff pistons (18, 118) that are movable axially within the internal cavity (22) against the action of the associated elastic element (19), - Sliding contact seal elements (23, 24, 123, 124) are positioned at the interface between the cutoff pistons (18, 118) and the internal cavity (22), defining an axially movable hydraulic chamber (15, 115) between the cutoff pistons (18, 118) and the internal cavity (22), Equipped with, The cutoff pistons (18, 118) are movable between two axial positions: The two axial positions are, The cutoff pistons (18, 118) compress the elastic element (19) and move the hydraulic chambers (15, 115) to a first axial position where the hydraulic chambers (15, 115) are in fluid communication with the expansion chamber (33) and the outlet port (14), but not with the inlet pipe (20), and The elastic element (19) is released, and the cutoff pistons (18, 118) are positioned in a second axial position in which the hydraulic chambers (15, 115) are in fluid communication with the outlet port (14) and the inlet pipe (20), but not with the expansion chamber (33). A hydraulic chamber (115) that is movable in the axial direction is extendable in the axial direction. - Laterally restricted intermediate portion (128) of the cutoff piston (118); - A movable annular lip seal (123) mounted on the restricted intermediate portion (128) of the cutoff piston (118) and slidingly contacting the cylindrical surface of the internal cavity (22); - A portion (129) of the cylindrical surface of the cutoff piston (118); - A static annular lip seal (124) attached to the laterally enlarged portion (122) of the internal cavity (22), positioned axially intermediate between the outflow channel (31) and the inflow channel (20), and making sliding contact with the cylindrical surface (129) of the cutoff piston (118); and - Part of the internal cavity (22), A valve unit defined by [the specified method / method].

2. A hydraulic chamber (15) that is movable in the axial direction, Two annular seals (23, 24) are mounted axially spaced on the cutoff piston (18) and slide against the cylindrical surface of the internal cavity (22); A laterally restricted intermediate portion (28) of the cutoff piston (18, 118) is formed between two annular seals (23, 24) on the cutoff piston 18; and The valve unit according to claim 1, defined by the cylindrical surface of the internal cavity (22).

3. The valve unit according to claim 2, wherein the two annular seals (23, 24) are O-ring annular seals.

4. The valve unit according to claim 2, further comprising third and fourth annular seals (25, 26) attached to a cutoff piston (18) and positioned axially outward from the two annular seals (23, 24).

5. At least one transverse passage (130a, 130b) extending into the piston and opening with at least two axially spaced openings (131, 132) is formed in the cutoff piston (118), The valve unit according to claim 1, wherein the first opening (131) is located on a restricted intermediate portion (128) of the piston, close in the axial direction to the outflow passage (31), and the second opening (132) is located on the cylindrical surface (129) of the piston, farther in the axial direction from the outflow passage (31).

6. The valve unit according to claim 5, comprising a plurality of interconnected transverse passages (130a, 130b) extending into the interior of a cutoff piston (118) and opening at one or more positions on a restricted intermediate portion (128) and on the cylindrical surface (129) of the piston, spaced axially from the restricted portion (128).

7. The valve unit according to claim 5, comprising at least one transverse flow path having an inclined portion (130a) and at least one radial portion (130b), wherein the inclined portion (130a) and the radial portion (130b) merge in the internal region of the cutoff piston (118).

8. Each of the cutoff pistons (118) and the body (11) is equipped with third and fourth O-ring seals (125, 126) positioned adjacent to the lip seals (123, 124) and axially outward, respectively. The valve unit according to any one of claims 1 to 7, wherein an O-ring seal (125) acts to slide against an internal cavity (22), and an O-ring seal (126) acts to slide against a cylindrical surface (129) of a cutoff piston (118).

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