A valve unit for an Anti-lock braking system for bicycles

Abstract

A valve unit for an anti-lock braking system for bicycles The valve unit (20) comprises an outer valve body (21) defining an axially elongate inner cavity (22) in which a piston valve member (23) is slidably driven. Three stationary annular sealing gaskets (31, 32, 33) are mounted on the outer valve body in the inner cavity (22) and slidingly sealingly engaging the piston valve member (23): an upstream end sealing gasket (31), a downstream end sealing gasket (32) and an intermediate sealing gasket (33) mounted within the inner cavity (22) between an inlet port (27) and an outlet port (26). The outer valve body (21) does not have to provide a sliding surface for moving sealing gaskets and may be made of plastic material.

Description

[0001] A valve unit for an anti-lock braking system for bicycles

[0002] Technical field

[0003] The present invention relates to a valve unit for a hydraulic braking system for controlling the anti-lock function of a wheel of a vehicle, particularly a bicycle or an e-bike.

[0004] Background art

[0005] Anti-Lock Braking Systems (“ABS”) have been installed on vehicles with hydraulic brakes to prevent skidding, by reducing the effects of an abrupt stop. In an ABS braking system, all wheels of a vehicle are equipped with brake discs and associated phonic wheel sensors or equivalent elements, rotationally integral with the brake discs. The sensors detect the rotation speeds of the wheels to which they are associated and send signals indicative of the rotation speeds to an Electronic Control Unit (ECU) which processes the rotation signals received. Each brake disc is associated with a brake caliper. A master cylinder, actuated by a control (hand lever control in a bicycle) activates the brake calipers through respective hydraulic circuit, on each of which an ABS valve unit is installed. Each ABS valve unit controls the flow and pressure of the brake fluid towards the associated brake caliper, in response to electrical control signals from the electronic control unit. When the ECU detects a condition indicative of an imminent wheel locking, it operates the respective ABS valve to reduce the hydraulic pressure on the brake at the affected wheel, thus reducing the braking force on this wheel, so that the wheel remains braked but can rotate. This process is repeated continuously during braking, several times per second, preventing the vehicle from skidding.

[0006] WO 2021 / 205337 Al discloses an ABS actuator device for a bicycle hydraulic braking system comprises an inner cavity in which a piston valve member is slidably accommodated, separating an upstream chamber from a downstream chamber. An inlet opening establishes fluid communication between the upstream chamber and a hydraulic master cylinder operated by a brake lever. An outlet opening connects the downstream chamber with a hydraulic slave cylinder acting on a brake calliper. The position of a piston valve member is controlled by an electric motor. During normal braking, in which there is no need for the ABS intervention, the electric motor is inactive, and the piston valve member is in an end position in the direction of the downstream chamber. A sealing ring mounted on the piston valve member floating member leaves an annular gap open between the sealing ring and the wall of the cavity, so that the gap connects the upstream chamber with the downstream chamber and the brake fluid pumped by the master cylinder can flow past the piston valve member in the direction of the brake calliper. In conditions that require activation of the ABS, the electric motor is activated, and causes the piston valve member to move towards the upstream chamber, so that the sealing ring engages a portion of reduced diameter of the cavity, interrupting the fluid communication between the upstream and downstream chambers. In this condition, the downstream chamber increases in volume, causing a decrease in the pressure of the brake fluid supplied to the brake calliper.

[0007] Summary of the invention

[0008] A primary object of the present invention is to provide an improved and simplified ABS valve unit, having a simplified design, easier to manufacture and assemble.

[0009] Another specific object of the present invention is to provide a valve unit which is more economical to manufacture, with a valve body in plastic material.

[0010] A further object of the invention is to provide a valve unit having safety features that ensure effective braking action even in the event of a failure or loss of electric power on an e-bike.

[0011] According to an aspect, the present invention provides a valve unit for a hydraulic braking system to control the anti-lock function of a vehicle wheel, as defined in claim 1. Preferred embodiments are defined in the dependent claims.

[0012] In summary, a valve unit for an anti-lock hydraulic braking system for a bicycle comprises an outer valve body defining an inner cavity having an axially elongate shape. A piston valve member is axially slidably accommodated within the inner cavity, and separates an upstream chamber from a downstream chamber within the inner cavity. The piston valve member is being connectable to an electric motor for controlling its axial position along the inner cavity. An inlet port is formed in the outer valve body for establishing fluid communication between the upstream chamber and a brake operating hydraulic master cylinder. An outlet port is also formed in the outer valve body, for establishing fluid communication between the downstream chamber and a brake calliper operatively associated with the valve unit. The inlet port and the outlet port are axially spaced from one another along the inner cavity. The piston valve member has a downstream end section with a given diameter, an upstream end section, and a central section with a smooth cylindrical surface having a diameter greater than that of the downstream end section. Formed in the central section are one or more radially narrower surfaces axially extending from the smooth cylindrical surface towards the upstream end section. Three stationary annular sealing gaskets are mounted on the outer valve body within the inner cavity, slidingly sealingly engaging the piston valve member: (i) an upstream end sealing gasket engaging the upstream end section; (ii) a downstream end sealing gasket engaging the downstream end section, and (iii) an intermediate sealing gasket mounted within the inner cavity between the inlet port and the outlet por. The upstream and downstream end sealing gaskets are arranged axially spaced from one another such that the inlet port and the outlet port are axially located between the upstream end and the downstream end sealing gaskets. The piston valve member is axially drivable between a normal braking position and at least one fluidically insulating position. In the normal braking position, the smooth cylindrical surface is axially shifted with respect to and is not engaged by the intermediate sealing gasket, and the intermediate sealing gasket is transversally aligned with and radially separated from the one or more radially narrower surfaces, whereby one or more corresponding passages are defined between the piston valve member and the intermediate sealing gasket, thereby establishing fluid communication between the downstream and upstream chambers. In the at least one fluidically insulating position, the piston valve member is axially shifted toward the upstream chamber, and the intermediate sealing gasket engages the smooth cylindrical surface, thereby fluidically insulating the downstream chamber from the upstream chamber.

[0013] Brief description of the drawings

[0014] In order that the present invention may be well understood there will now be described a few preferred embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which:

[0015] Figure 1 is a diagram schematically depicting the operation of an anti-lock braking system on a vehicle;

[0016] Figures 2 to 4 are longitudinal cross-sectional views of a valve unit in different operating conditions;

[0017] Figure 5 is a partial enlarged view of Figure 4, showing further details;

[0018] Figure 6 is an enlarged transversal cross-sectional view of a piston valve member of the valve unit of Figures 2-5; and

[0019] Figure 7 is an enlarged view of a detail of Figure 2.

[0020] Detailed description

[0021] Referring initially to Figure 1, an Anti-Lock Braking System (ABS) comprises wheel rotation sensors 10 applied to the wheels of a vehicle, in this example an e-bike. The wheels are each equipped with brake discs (not shown) and associated brake calipers 11, which apply braking force to brake pads on the caliper through a hydraulic brake circuit 15 and consequently generate a braking torque acting on the wheel. A hand lever 12 operates a hydraulic master cylinder 12 for generating and controlling pressure within the hydraulic brake circuit 15. A pressure sensor 14 detects the brake fluid pressure in the hydraulic circuit. The rotation sensors 10 detect the rotational speeds of the wheels to which they are associated and send signals indicative of the rotation speeds to a Brake Control Unit (BBCU) 16, which is an electronic unit that receives and processes signals from the rotation sensors 10 and the pressure sensor 14. Electric signals from the pressure sensor 14 are sent to the BBCU 16 through a line 9.

[0022] Each brake caliper is associated with an ABS valve unit 20. Each ABS valve unit controls the flow and pressure of the brake fluid towards the associated brake caliper, in response to electrical control signals from the Brake Control Unit 16. When the BBCU detects a condition indicative of an imminent wheel locking, it operates the respective ABS valve unit in order to reduce the hydraulic pressure on the brake at the affected wheel, thus reducing the braking force on this wheel, so that the wheel remains braked but can rotate. This process is repeated continuously during braking, several times per second, preventing the vehicle from skidding.

[0023] Referring to Figure 2, each ABS valve unit 20 comprises an outer valve body (or housing) 21 defining an inner cavity 22 having an axially elongate shape. A piston valve member 23 is axially slidably accommodated within the inner cavity, separating an upstream chamber 25 from a downstream chamber 24. The outer valve body 21 forms an inlet port 27 establishing fluid communication between the upstream chamber 25 and the hydraulic master cylinder 13 operated by the brake lever 12. An outlet port 26 fluidically connects the downstream chamber 24 with one brake caliper 11 associated with the valve unit 20. The inlet port 27 and the outlet port 26 are axially spaced from one another along the inner cavity 22. In this context, the expressions “upstream” and “downstream” are referred to the flow of brake fluid from the master cylinder to the brake caliper.

[0024] An electric motor 28, secured to the outer valve body 21, controls the axial position of the piston valve member 23. The electric motor 28 drives a rotating output shaft 29 coupled to the piston valve member 23 through a rotary to linear motion conversion mechanism 30 that converts rotary motion of the rotating shaft into linear displacement of the piston valve member 23 within the inner cavity 22. The mechanism 30 may include a threaded coupling. The electric motor 28 is energized and controlled by the BBCU 16 through a line 8. The BBCU 16 receives power from a battery (not shown) through a line 7.

[0025] The electric motor may be provided with a position sensor for detecting the angular position of the output shaft 29, and consequently, detecting the axial position of the piston valve member 23.

[0026] According to an embodiment, a first or upstream hollow tubular insert 35 is fitted within the inner cavity 22 and suitably shaped to fit two sealing gaskets within the outer valve body 21 and provide an axial passage for guiding an upstream end section 231 of the piston valve member 23, as well as establishing fluid communication between the upstream chamber 25 and the inlet port 27.

[0027] Mounted on the outer valve body 21 within the inner cavity 22 are three stationary annular sealing gaskets 31-33, slidingly and sealingly engaging the piston valve member 23: a first end (or upstream end) sealing gasket 31 and a second end (or downstream end) sealing gasket 32, which are arranged axially spaced from one another such that the inlet port 27 and the outlet port 26 are axially located between the upstream end and the downstream end sealing gaskets 31, 32. A third, intermediate sealing gasket 33 is mounted within the inner cavity between the inlet port 27 and the outlet port 26. No sealing gaskets are mounted on the piston valve member 23.

[0028] The third, intermediate sealing gasket 33 can fluidically separate the downstream and upstream chambers 24, 25 and, depending on the axial position of the piston valve member

[0029] 23, can open and close a passage between the piston valve member and the inner cavity, thereby establishing fluid communication between the downstream and upstream chambers

[0030] 24, 25 or temporarily interrupting the fluid communication between the outlet and inlet ports 26, 27 by sealingly separating the upstream chamber from the downstream chamber.

[0031] According to an embodiment, a second or downstream hollow tubular insert 36 is fitted within the inner cavity 22 and suitably shaped to fit the downstream sealing gasket 32 within the outer valve body 21 and provide an axial passage for guiding a downstream end section 232 of the piston valve member 23.

[0032] The upstream chamber 25, communicating with the inlet port 27, is delimited between the upstream end sealing gasket 31 and the intermediate sealing gasket 33. The downstream chamber 24, communicating with the outlet port 26, is delimited between the downstream end sealing gasket 32 and the intermediate sealing gasket 33.

[0033] The upstream and downstream hollow tubular inserts 35, 36 are hermetically sealingly coupled to the outer valve body 21 by a number of static sealing rings such as O-rings 39 acting at the interfaces between each insert and the inner cavity 22.

[0034] The piston valve member 23, on which the intermediate sealing gasket 33 can exert a sliding or dynamic sealing action, may be made in metal, for example an aluminum alloy, or another suitable rigid material for example a thermosetting plastic material. The outer valve body 21 has the task of steadily accommodating the sealing gaskets, but the wall of the inner cavity 22 does not also have the task of acting as a smooth sliding surface for a gasket, capable of withstanding high pressures without deforming noticeably. Consequently, the outer valve body may be conveniently made of a relatively inexpensive thermoplastic material, such as, for example, PA 66 GF30 - PA 66 GF 50 - PA 6 - PA 66 CF30.

[0035] In accordance with the exemplary embodiment shown in Figure 2, the outer valve body 21 is shaped as a substantially tubular body with axially opposite open ends and a central, radially thicker wall portion 211 in which the outlet port 26 may be formed.

[0036] As shown in the embodiment of Figure 2, the central, radially thicker wall portion 211 forms an upstream transversal surface 212 against which the intermediate sealing gasket can be steadily held in position between the central wall portion 211 and the upstream hollow tubular insert 35.

[0037] Preferably, the upstream sealing gasket 31 is steadily accommodated in an annular seat formed by the upstream end hollow tubular insert 35 and an upstream end plug 38 that may be used to close the upstream open end of the outer valve body 21 and slidingly accommodate the upstream end section 231 of piston valve member 23.

[0038] According to a preferred embodiment, the downstream sealing gasket 32 may be steadily accommodated in an annular seat formed by the downstream end hollow tubular insert 36 and a downstream transversal surface 213 formed by the central, radially thicker wall portion 211 opposite to the upstream transversal surface 212.

[0039] The three stationary annular sealing gaskets 31-33 are each preferably shaped with a respective conical lip designed for elastically radially engaging a respective surface of the piston valve member 23. The conical lip of the upstream end sealing gasket 31 tapers towards the upstream chamber 25, whereas the conical lip of the downstream end sealing gasket 32 tapers towards the downstream chamber 24.

[0040] Preferably, the conical lip of the intermediate sealing gasket 33 tapers towards the upstream chamber 25.

[0041] The piston valve member 23 has a central section 233 with a smooth cylindrical surface 235 having a diameter D3 and one or more radially narrower surfaces 234 axially extending from the cylindrical surface 235 towards the upstream end of the piston valve member. The one or more radially narrower surfaces 234 are configured to define one or more corresponding passages 236 between the piston valve member 23 and the intermediate sealing gasket 33 in certain operational conditions of the valve unit.

[0042] According to an embodiment, as show in Figure 6, the radially narrower surfaces 234 may be provided in the form of axially extending and circumferentially equally spaced grooves.

[0043] According to a preferred embodiment, the piston valve member 23 provides the central section 233 in a substantially central position along its axial length.

[0044] The downstream end section 232 of the piston valve member 23 has a diameter D2 that is smaller than the diameter D3 of the central section 233.

[0045] Preferably, the upstream end section 231 of the piston valve member 23 has a diameter DI that is smaller than the diameter D3 of the central section 233.

[0046] Still more preferably, the upstream end section 231 of the piston valve member 23 has a diameter DI that is smaller than the diameter D2 of the downstream end section 232.

[0047] Optionally, the Brake Control Unit 16 (BBCU) may have an integrated Inertial Measurement Unit (IMU), designated at 6, which detects acceleration and orientation of the e-bike. Based on these measurements, the BBCU can energize the electric motor to control the position of the piston valve member 23, activating the valve unit in order to prevent roll-over of the bike when the rider brakes excessively on high friction road surfaces, and prevents locking of the controlled wheel when the rider brakes on low friction surfaces.

[0048] In the following, exemplary operating modes of the ABS valve will be described: a normal braking operating mode, a fluidically insulated condition, and a pressure modulation operating mode.

[0049] During normal operation of the hydraulic bicycle brake, the ABS valve is in an idle condition (Figure 2). The piston valve member 23 is retracted in a start position, toward the downstream chamber 24. In this position, the free end of the conical lip of the intermediate sealing gasket 33 is in an open position (Figure 7), in which the smooth cylindrical surface 235 is axially shifted with respect to and is not engaged by the intermediate sealing gasket 33. In this open or normal operation position, the intermediate sealing gasket 33 is transversally aligned with and radially separated from the one or more radially narrower surfaces 234. As a result, the one or more corresponding passages 236 defined between the piston valve member 23 and the intermediate sealing gasket 33 establish fluid communication between the downstream and upstream chambers 24, 25, and therefore between the master cylinder and the brake caliper.

[0050] During normal operation of the brake, the piston valve member 23 remains in such a retracted, passive position and the brake fluid pumped by the brake lever 12 can flow from the inlet port 27 to the outlet port 26, through the passages or grooves 234, and then to the brake caliper 11 without any kind of interference by the ABS system. In this operating mode, the BBCU control unit 16 constantly monitors the pressure in the hydraulic system, the angular speed of the wheels and, optionally, also information from the IMU. A control algorithm processes this information by deciding to switch to one of the remaining listed operating modes.

[0051] In locked wheel braking conditions, or when a condition of impending rollover or other loss of control during braking is perceived, the BBCU 16 energizes the control motor 28 of the wheel concerned, causing the relevant piston valve member associated with that wheel to slide towards the upstream chamber 25 (Figure 3), away from the initial inactive position of Figure 2. The piston valve member 23 reaches an axially shifted position toward and the upstream chamber 25, in which the free end of the conical lip of the intermediate sealing gasket 33 is in a closed position, sealingly engaging the smooth cylindrical surface 235 of central section 233 of the piston valve member 23. In this position of the piston valve member (Figure 3), the intermediate sealing gasket 33 fluidically insulates the downstream chamber 24 from the upstream chamber 25. As a result, fluid communication between the master cylinder and the brake calliper, is temporarily interrupted.

[0052] This fluidically insulated condition is a transient condition because the electric motor 28 drives the piston valve member further into the upstream chamber 25 (Figure 4). Since the downstream end section 232 of the piston valve member 23 has a diameter D2 that is smaller than the diameter D3 of the central section 233 (Figure 5), the movement of the piston valve member 23 towards or further into the upstream chamber 25 causes the downstream chamber 24 to increase in volume, thereby instantaneously releasing the pressure of the brake fluid in communication with the brake calliper.

[0053] According to a pressure modulation operating mode, the BBCU 16 controls the electric motor 28 so as to drive the piston valve member 23 back and forth repeatedly several times per second during braking, whereby the position of the piston valve member 23 performs linear reciprocating motion between the fluid insulated condition of Figure 3, in which the intermediate sealing gasket 33 seals the downstream chamber 24 from the upstream chamber 25, and the position of Figure 4, in which the intermediate sealing gasket 33 still seals the downstream chamber 24 from the upstream chamber 25, but the piston valve member 23 is farther into the upstream chamber, thereby increasing the volume and reducing the pressure in the downstream chamber 24.

[0054] As a result, the braking torque intermittently transmitted to the brake caliper prevents wheel locking, or rollover.

[0055] It will be observed that during the pressure modulation operating mode, the downstream chamber 24 remains fluidically separated or sealed from the upstream chamber 25, whereby the master cylinder cannot exert any action on the brake caliper.

[0056] The BBCU controls the electric motor and the piston valve member in the pressure modulation mode until the BBCU, through the hydraulic pressure sensor, detects that the pressure in the downstream chamber falls below a predetermined threshold, and sends a control signal to the electric motor 28, causing the piston valve member 23 to return to its starting position (Figure 2), switching back to the normal operating mode of the ABS valve.

[0057] Due to the design and position of the intermediate sealing gasket 33, fluidic insulation or sealing action performed by the intermediate sealing gasket 33 is maintained insofar as the pressure in the upstream chamber 25 is greater than or equal to the pressure in the downstream chamber 24. Conversely, when the pressure in the upstream chamber 25 becomes less than the pressure in the downstream chamber 24, which occurs when the rider releases the brake lever, the intermediate sealing gasket 33 allows the brake fluid to flow back from the downstream chamber 24 to the upstream chamber 25. This capability prevents braking torque from being applied to the brake calliper even when the rider releases the brake.

[0058] An algorithm manages the full axial movement of the piston up to reaching the insulating operating mode with the intermediate sealing gasket arranged as shown in Figure 3, in which the brake calliper is fluidically insulated with respect to the master cylinder. If this condition is still not sufficient to effectively control braking, the algorithm acts to adjust the pressure within the downstream chamber in accordance with the above-described pressure modulation operating mode.

[0059] As an alternative to the above-described pressure modulation operating mode, the valve unit 20 may be controlled in a more conventional operating mode. According to the alternative operating mode, upon detecting a critical (skidding or rollover) braking condition, the BBCU energizes the electric motor 28 to drive the piston valve member 23 from the initial position of Figure 2 to the sealed or insulated position of Figure 4, in which the pressure in the hydraulic circuit between the intermediate sealing gasket 33 and the brake calliper is released. As soon as the BBCU, through the hydraulic pressure sensor, detects that the pressure in the downstream chamber falls below a predetermined threshold, it sends a control signal to the electric motor 28, causing the piston valve member 23 to return to its starting position (Figure 2), thereby reopening the flow of brake fluid through the passages or grooves 234, and temporarily re-establishing direct fluid communication between the master cylinder and the brake calliper. Upon detecting a persisting critical braking condition, the BBCU will immediately drive the piston valve member back to the sealed or insulated position of Figure 4, and this process of opening and closing the passages 234 is repeated continuously during braking, several times per second, preventing the vehicle from skidding.

[0060] The arrangement of diameters D3 > D2 > DI of sections 233, 232, and 231, respectively, of the piston valve member 23 provides also an advantageous safe failure condition. In the event of a power failure of the electric motor 28 while the piston 23 is in the position of Figure 4, although the intermediate sealing gasket 33 fluidically insulates the master cylinder from the brake calliper, the diameter D3 of the central section 233, being wider than the diameter DI of the upstream end section 231, causes the piston valve member 23 to move (to the right) toward the downstream chamber 24, until the passages 234 are reopened.

[0061] Preferably, the diameter D2 of the downstream end section 232 is greater than the diameter DI of the upstream end section 231. As a result, as soon as the passages 234 are reopened, since the same hydraulic pressure will be present in the downstream chamber and the upstream chamber, the piston valve member 23 will be effectively urged to move (to the right) toward the downstream chamber 24. This is because the area of an annulus delimited by the diameters DI and D3 (on the upstream chamber side) is greater than the area of an annulus delimited by the diameters D2 and D3 (on the downstream chamber side). Consequently, the hydraulic forces acting on opposite sides of the piston valve member 23 will provide an axial net force pushing it towards and further into the downstream chamber 24, reaching the position of Figure 2. As a result, the valve unit always guarantees a sufficient braking torque to allow the vehicle to stop in a reasonable stopping distance, even in the event of an electric failure.

[0062] While specific embodiments of the invention have been disclosed, it is to be understood that such disclosure has been merely for the purpose of illustration and that the invention is not to be limited in any manner thereby. Various modifications will be apparent to those skilled in the art in view of the foregoing examples. The scope of the invention is to be limited only by the appended claims.

Claims

CLAIMS1. A valve unit (20) for an anti-lock hydraulic braking system for a bicycle, the valve unit comprising: an outer valve body (21) defining an inner cavity (22) having an axially elongate shape; a piston valve member (23) axially slidably accommodated within the inner cavity(22), and separating an upstream chamber (25) from a downstream chamber (24) within the inner cavity (22), the piston valve member (23) being connectable to an electric motor (28) for controlling the axial position of the piston valve member (23) along the inner cavity (22); an inlet port (27) formed in the outer valve body (21), for establishing fluid communication between the upstream chamber (25) and a brake operating hydraulic master cylinder (13); an outlet port (26) formed in the outer valve body (21), for establishing fluid communication between the downstream chamber (24) and a brake caliper (11) operatively associated with the valve unit (20), wherein the inlet port (27) and the outlet port (26) are axially spaced from one another along the inner cavity (22); the piston valve member (23) having a downstream end section (232) with a diameter (D2), an upstream end section (231), and a central section (233) with a smooth cylindrical surface having a diameter (D3) and one or more radially narrower surfaces (234) axially extending from the smooth cylindrical surface (235) towards the upstream end section (231), wherein the diameter (D3) of the central section (233) is greater than the diameter (D2) of the downstream end section (232); three stationary annular sealing gaskets (31, 32, 33) mounted on the outer valve body (21) within the inner cavity (22) and slidingly sealingly engaging the piston valve member(23):(i) an upstream end sealing gasket (31) engaging the upstream end section (231);(ii) a downstream end sealing gasket (32) engaging the downstream end section (232), wherein the upstream and downstream end sealing gaskets (31, 32) are arranged axially spaced from one another such that the inlet port (27) and the outlet port (26) are axially located between the upstream end and the downstream end sealing gaskets (31, 32);(iii) an intermediate sealing gasket (33) mounted within the inner cavity (22) between the inlet port (27) and the outlet port (26),wherein the piston valve member (23) is axially drivable between a normal braking position, in which the smooth cylindrical surface (235) is axially shifted with respect to and is not engaged by the intermediate sealing gasket (33), and the intermediate sealing gasket (33) is transversally aligned with and radially separated from the one or more radially narrower surfaces (234), whereby one or more corresponding passages (236) are defined between the piston valve member (23) and the intermediate sealing gasket (33), thereby establishing fluid communication between the downstream and upstream chambers (24, 25); and at least one fluidically insulating position, in which the piston valve member (23) is axially shifted toward the upstream chamber (25), and the intermediate sealing gasket (33) engages the smooth cylindrical surface (235), thereby fluidically insulating the downstream chamber (24) from the upstream chamber (25).

2. A valve unit according to claim 1, wherein the outer valve body (21) defining the inner cavity (22) is made of plastic material.

3. A valve unit according to claim 1 or claim 2, wherein the upstream end section (231) of the piston valve member (23) has a diameter (DI) that is smaller than the diameter (D3) of the central section (233).

4. A valve unit according to claim 3, wherein the diameter (D2) of the downstream end section (232) is greater than the diameter (DI) of the upstream end section (231).

5. A valve unit according to any one of the preceding claims, wherein the intermediate sealing gasket (33) has a conical lip that tapers towards the upstream chamber (25).

6. A valve unit according to claim 1, wherein the one or more radially narrower surfaces (234) comprise axially extending and circumferentially spaced grooves.

7. A valve unit according to claim 1, wherein the outer valve body (21 ) is shaped as a substantially tubular body with axially opposite open ends and a central, radially thicker wall portion (211) providing a transversal surface(212) facing the upstream chamber (25), an upstream hollow tubular insert (35) is fitted within the inner cavity (22), and provides an axial passage for accommodating the upstream end section (231) of the piston valve member (23), and wherein the intermediate sealing gasket (33) is steadily held in an axial position between the central wall portion (211) and the upstream hollow tubular insert (35).

8. A valve unit according to any one of the preceding claims, wherein the valve unit is configured so that the piston valve member (23) can be driven, in accordance with a pressure modulation operational mode, to perform linear reciprocating motion and reach repeatedly and alternately two relatively axially displaced fluidically insulating positions: a first fluidically insulating position, in which the piston valve member (23) is axially shifted toward the upstream chamber (25), and the intermediate sealing gasket (33) engages the smooth cylindrical surface (235), thereby fluidically insulating the downstream chamber (24) from the upstream chamber (25); and a second fluidically insulating position, in which the piston valve member (23) is axially shifted farther toward and into the upstream chamber (25), and the intermediate sealing gasket (33) engages the smooth cylindrical surface (235), thereby fluidically insulating the downstream chamber (24) from the upstream chamber (25).

9. A valve unit according to any one of claims 1 to 7, wherein the valve unit is configured so that the piston valve member (23) can be driven, in accordance with an operational mode, to perform linear reciprocating motion and reach repeatedly and alternately two relatively axially displaced positions: said normal braking position, in which the smooth cylindrical surface (235) is axially shifted with respect to and is not engaged by the intermediate sealing gasket (33), and the intermediate sealing gasket (33) is transversally aligned with and radially separated from the one or more radially narrower surfaces (234), whereby one or more corresponding passages (236) are defined between the piston valve member (23) and the intermediate sealing gasket (33), thereby establishing fluid communication between the downstream and upstream chambers (24, 25); and said at least one fluidically insulating position, in which the piston valve member (23)is axially shifted toward the upstream chamber (25), and the intermediate sealing gasket (33) engages the smooth cylindrical surface (235), thereby fluidically insulating the downstream chamber (24) from the upstream chamber (25).

10. An anti-lock hydraulic braking system for a bicycle, comprising a valve unit (20) according to any one of the preceding claims.

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