Brake unit for anti-lock brake system of bicycle

Abstract

A valve unit for a bicycle anti-lock brake system. The valve unit (20) comprises an outer valve body (21) defining an axially elongated inner cavity (22) in which a piston valve member (23) is slidably driven. Three stationary annular sealing gaskets (31, 32, 33) mounted on the outer valve body within the inner cavity (22) and in sliding sealing engagement with the piston valve member (23); an upstream end sealing gasket (31), a downstream end sealing gasket (32), and an intermediate sealing gasket (33) are mounted within the interior 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 the sealing gasket, and the outer valve body (21) may be made of a plastic material.

Description

Technical Field

[0001] This invention relates to a valve unit for controlling the anti-lock braking function of a vehicle wheel in a hydraulic braking system, particularly for bicycles or electric bicycles. Background Technology

[0002] Anti-lock Braking System (ABS) is installed on vehicles with hydraulic brakes to reduce the impact of sudden stops and prevent skidding. In an ABS braking system, all wheels of the vehicle are equipped with brake discs and their associated phonic wheel sensors or equivalent components, which rotate integrally with the brake discs. Sensors detect the rotational speed of their associated wheels and transmit signals indicating these speeds to an Electronic Control Unit (ECU), which processes the received rotational signals. Each brake disc is associated with a brake caliper. A master cylinder, actuated by a controller (e.g., handlebar control in a bicycle), activates the brake caliper via a corresponding hydraulic circuit, each circuit housing an ABS valve unit. Each ABS valve unit responds to electronic control signals from the ECU, controlling the flow and pressure of brake fluid toward the associated brake caliper. When the ECU detects an indication that wheel lock-up is imminent, it operates the corresponding ABS valve to reduce the brake fluid pressure on the affected wheel, thereby reducing the braking force on that wheel and keeping it braked but still rotatable. This process continues during braking, several times per second, to prevent the vehicle from skidding.

[0003] WO 2021 / 205337 A1 discloses an ABS actuator device for a bicycle hydraulic braking system, comprising an inner cavity in which a piston valve member is slidably received, separating an upstream chamber and 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 to a hydraulic slave cylinder acting on a brake caliper. The position of the piston valve member is controlled by an electric motor. During normal braking, without 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 a floating component of the piston valve member leaves an open annular gap between the sealing ring and the cavity wall, thereby connecting the upstream and downstream chambers, and allowing brake fluid pumped by the master cylinder to flow through the piston valve member in the direction of the brake caliper. When ABS activation is required, the electric motor is activated, causing the piston valve member to move towards the upstream chamber, causing the sealing ring to engage with a portion of the reduced diameter of the cavity, thereby interrupting the fluid communication between the upstream and downstream chambers. In this situation, the volume of the downstream chamber increases, resulting in a decrease in the pressure of the brake fluid supplied to the brake caliper. Summary of the Invention

[0004] The main objective of this invention is to provide an improved and simplified ABS valve unit that is simpler in design and easier to manufacture and assemble.

[0005] Another specific object of the present invention is to provide a valve unit with a more economical manufacturing cost, which has a valve body made of plastic material.

[0006] A further object of the present invention is to provide a valve unit with safety features that ensures effective braking even in the event of a malfunction or power loss in an electric bicycle.

[0007] According to one aspect, the present invention provides a valve unit for controlling a hydraulic braking system for controlling the anti-lock braking function of vehicle wheels, as defined in claim 1. Preferred embodiments are defined in the appended claims.

[0008] In summary, a valve unit for an anti-lock braking system (ABS) in a bicycle includes an outer valve body defining an inner cavity having an axially elongated shape. A piston valve member is axially slidably received within the inner cavity and divides the cavity into an upstream chamber and a downstream chamber. The piston valve member is 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 caliper operatively associated with the valve unit. The inlet and outlet ports are axially spaced apart from each other along the inner cavity. The piston valve member has a downstream end section, an upstream end section, and a central section. The downstream end section has a given diameter, and the central section has a smooth cylindrical surface and a diameter larger than that of the downstream end section. One or more radially narrow surfaces are formed in the central section, and the one or more radially narrow surfaces extend axially from the smooth cylindrical surface towards the upstream end section. Three stationary annular gaskets are mounted on the outer valve body within the inner cavity and slidably seal the piston valve assembly: (i) an upstream end gasket that engages the upstream end section; (ii) a downstream end gasket that engages the downstream end section; and (iii) an intermediate gasket mounted within the inner cavity between the inlet and outlet ports. The upstream and downstream end gaskets are axially spaced apart from each other such that the inlet and outlet ports are axially located between the upstream and downstream end gaskets. The piston valve assembly is axially actuated between a normal braking position and at least one fluid-isolated position. In the normal braking position, the smooth cylindrical surface is axially offset relative to and does not engage the intermediate gasket, and the intermediate gasket is laterally aligned and radially separated from the one or more radially narrow surfaces, defining one or more corresponding channels between the piston valve assembly and the intermediate gasket, thereby establishing fluid communication between the downstream and upstream chambers. In at least one fluid isolation position, the piston valve component is axially offset towards the upstream chamber, and the intermediate sealing gasket engages a smooth cylindrical surface, thereby fluidly isolating the downstream chamber from the upstream chamber. Attached Figure Description

[0009] To better understand the present invention, several preferred embodiments will be described below, illustrated by way of example only with reference to the accompanying drawings, wherein: Figure 1 It is a schematic diagram illustrating the operation of an anti-lock braking system on a vehicle; Figures 2 to 4 It is a longitudinal cross-sectional view of a valve unit under different operating conditions; Figure 5 yes Figure 4 A magnified view of the portion, showing further details; Figure 6 yes Figures 2 to 5 An enlarged cross-sectional view of the piston valve component of a valve unit is shown; and Figure 7 yes Figure 2 A magnified view of a detail. Detailed Implementation

[0010] First refer to Figure 1 An anti-lock braking system (ABS) includes wheel rotation sensors 10 applied to the wheels of a vehicle (in this example, an electric bicycle). Each wheel is equipped with a brake disc (not shown) and an associated brake caliper 11. The brake caliper 11 applies braking force to the brake pads on the caliper via a hydraulic brake circuit 15, thereby generating a braking torque acting on the wheel. A handle 12 operates a hydraulic master cylinder 12 to generate and control the pressure within the hydraulic brake circuit 15. A pressure sensor 14 detects the brake fluid pressure in the hydraulic circuit. The rotation sensor 10 detects the rotational speed of its associated wheel and sends a signal indicating the rotational speed to a brake control unit (BBCU) 16, an electronic unit that receives and processes signals from the rotation sensor 10 and the pressure sensor 14. The electronic signal is transmitted from the pressure sensor 14 to the BBCU 16 via a line 9.

[0011] Each brake caliper is associated with an ABS valve unit 20. Each ABS valve unit, in response to an electronic control signal from the brake control unit 16, controls the pressure and flow of brake fluid to the associated brake caliper. When the BBCU detects a condition indicating that a wheel is about to lock up, it operates the corresponding ABS valve unit to reduce the brake fluid pressure on the affected wheel, thereby reducing the braking force on that wheel, allowing the wheel to remain braked but still rotatable. This process continues during braking, multiple times per second, to prevent vehicle skidding.

[0012] refer to Figure 2 Each ABS valve unit 20 includes an outer valve body (or housing) 21 defining an inner cavity 22 having an axially elongated shape. A piston valve member 23 is axially slidably received 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 a hydraulic master cylinder 13 operated by the brake handle 12. An outlet port 26 fluidly connects the downstream chamber 24 to a brake caliper 11 associated with the valve unit 20. The inlet port 27 and the outlet port 26 are axially spaced apart within the inner cavity 22. In this document, "upstream" and "downstream" refer to the flow of brake fluid from the master cylinder to the brake caliper.

[0013] An electric motor 28 is fastened to the external valve body 21 and controls the axial position of the piston valve component 23. The electric motor 28 drives a rotary output shaft 29 coupled to the piston valve component 23 via a rotation-to-linear motion conversion mechanism 30, which converts the rotational motion of the shaft into linear displacement of the piston valve component 23 within the inner cavity 22. The mechanism 30 may include threaded coupling. The electric motor 28 is powered and controlled by a BBCU 16 via a line 8. The BBCU 16 receives power from a battery (not shown) via a line 7.

[0014] The electric motor can be equipped with a position sensor to detect the angular position of the output shaft 29, and then detect the axial position of the piston valve component 23.

[0015] According to one embodiment, a first or upstream hollow tubular insert 35 is fitted within the inner cavity 22 and has a suitable shape to fit two gaskets within the outer valve body 21 and provide an axial passage to guide an upstream end section 231 of the piston valve component 23, while establishing fluid communication between the upstream chamber 25 and the inlet port 27.

[0016] Three stationary annular gaskets 31-33, mounted within the inner cavity 22 on the outer valve body 21, slidably and sealingly engage with the piston valve assembly 23: a first-end (or upstream) gasket 31 and a second-end (or downstream) gasket 32, axially spaced apart, such that the inlet port 27 and the outlet port 26 are located axially between the upstream gasket 31 and the downstream gasket 32. A third intermediate gasket 33 is installed within the inner cavity between the inlet port 27 and the outlet port 26. No gaskets are installed on the piston valve assembly 23.

[0017] The third intermediate sealing gasket 33 can fluidly separate the downstream chamber 24 from the upstream chamber 25, and can open and close a passage between the piston valve member and the inner cavity according to the axial position of the piston valve member 23, thereby establishing fluid communication between the downstream chamber 24 and the upstream chamber 25 or temporarily interrupting fluid communication between the outlet port 26 and the inlet port 27 by sealingly separating the upstream chamber and the downstream chamber.

[0018] According to one embodiment, a second or downstream hollow tubular insert 36 is fitted within the inner cavity 22 and has a suitable shape to fit the downstream sealing gasket 32 ​​within the outer valve body 21, and provides an axial passage to guide a downstream end portion 232 of the piston valve component 23.

[0019] The upstream chamber 25 is connected to the inlet port 27 and is confined between the upstream sealing gasket 31 and the intermediate sealing gasket 33. The downstream chamber 24 is connected to the outlet port 26 and is confined between the downstream sealing gasket 32 ​​and the intermediate sealing gasket 33.

[0020] The upstream hollow tubular insert 35 and the downstream hollow tubular insert 36 are sealed to the external valve body 21 at the interface between each insert and the inner cavity 22 by several stationary sealing rings (e.g., O-rings 39).

[0021] The intermediate sealing gasket 33 on the piston valve component 23 can provide a sliding or dynamic seal and can be made of metal, such as aluminum alloy, or other suitable rigid materials, such as a thermosetting plastic. The outer valve body 21 serves to stably house the sealing gasket, but the wall surface of the inner cavity 22 does not need to act as a sliding surface for the sealing gasket and can withstand high pressure without significant deformation. Therefore, the outer valve body can be conveniently made of relatively inexpensive thermoplastic materials, such as PA 66 GF30–PA 66 GF 50–PA6–PA 66 CF30.

[0022] according to Figure 2 In the example embodiment shown, the outer valve body 21 is shaped as a generally tubular body, having axially opposite open ends and a central, radially thicker wall 211 in which an outlet port 26 may be formed.

[0023] like Figure 2 In the embodiment shown, the central, radially thicker wall 211 forms an upstream transverse surface 212, and the intermediate sealing gasket can be stably maintained in position between the central wall 211 and the upstream hollow tubular insert 35.

[0024] Preferably, the upstream sealing gasket 31 is stably accommodated on an annular seat formed by an upstream hollow tubular insert 35 and an upstream plug 38 which can be used to close the upstream open end of the external valve body 21, and slidably accommodates the upstream end section 231 of the piston valve component 23.

[0025] According to a preferred embodiment, the downstream sealing gasket 32 ​​can be stably accommodated in an annular seat formed in a downstream hollow tubular insert 36 and a downstream transverse surface 213 formed by a centrally radially thickened wall portion 211 (which is opposite to the upstream transverse surface 212).

[0026] Three stationary annular gaskets 31-33 are preferably shaped individually with tapered lips, designed to resiliently and radially engage a corresponding surface of the piston valve component 23. The tapered lip of the upstream gasket 31 tapers towards the upstream chamber 25, while the tapered lip of the downstream gasket 32 ​​tapers towards the downstream chamber 24.

[0027] Preferably, the tapered lip of the intermediate sealing gasket 33 gradually tapers toward the upstream chamber 25.

[0028] The piston valve component 23 has a central section 233 with a smooth cylindrical surface 235 and one or more radially narrower surfaces 234. The central section 233 has a diameter D3, and the one or more radially narrower surfaces 234 extend axially from the cylindrical surface 235 toward the upstream end of the piston valve component. The one or more radially narrower surfaces 234 are configured to define one or more corresponding channels 236 between the piston valve component 23 and the intermediate sealing gasket 33 under certain operating conditions of the valve unit.

[0029] According to one embodiment, such as Figure 6 As shown, these radially narrow surfaces 234 can extend axially and are arranged in the form of equally spaced grooves around the circumference.

[0030] According to a preferred embodiment, a central section 233 is provided at approximately the center position of the piston valve component 23 along its axial length.

[0031] The downstream end section 232 of the piston valve component 23 has a diameter D2, which is smaller than the diameter D3 of the central section 233.

[0032] Preferably, the upstream end section 231 of the piston valve component 23 has a diameter D1, which is smaller than the diameter D3 of the central section 233.

[0033] More preferably, the upstream end section 231 of the piston valve component 23 has a diameter D1, which is smaller than the diameter D2 of the downstream end section 232.

[0034] Optionally, the brake control unit 16 (BBCU) may have an integrated inertial measurement unit (IMU), indicated by reference numeral "6," for detecting the acceleration and direction of the electric bicycle. Based on these measurements, the brake control unit may energize the electric motor to control the position of the piston valve member 23, activating the valve unit to prevent the bicycle from rolling over when the rider brakes excessively on high-friction surfaces, and to prevent the controlled wheel from locking up when the rider brakes on low-friction surfaces.

[0035] The following describes example operating modes of the ABS valve: a normal braking operating mode, a fluid isolation state, and a pressure regulating operating mode.

[0036] During normal operation of hydraulic bicycle brakes, the ABS valve is in an idle state. Figure 2 The piston valve assembly 23 is in its initial position, retracting towards the downstream chamber 24. In this position, the free end of the tapered lip of the intermediate sealing gasket 33 is in an open position. Figure 7The smooth cylindrical surface 235 is axially offset relative to the intermediate gasket 33 and is not engaged with the intermediate gasket 33 in the axial direction. In this open or normal operating position, the intermediate gasket 33 is laterally aligned with and radially separated from one or more radially narrower surfaces 234. Therefore, fluid communication is established between the downstream chamber 24 and the upstream chamber 25 between one or more corresponding channels 236 defined between the piston valve assembly 23 and the intermediate gasket 33, and thus also between the master cylinder and the brake caliper.

[0037] During normal braking operation, the piston valve assembly 23 remains in this contracted, passive position, allowing brake fluid pumped by the brake handle 12 to flow from inlet port 27 to outlet port 26, through channel or groove 234, and then to brake caliper 11, without any interference from the ABS system. In this operating mode, the BBCU 16 continuously monitors the pressure in the hydraulic system, the wheel angular velocity, and optionally, information from the IMU. A control algorithm determines whether to switch to one of the remaining listed operating modes to process this information.

[0038] Under braking conditions where the wheel is locked, or when a potential rollover or other loss of control is detected during braking, BBCU 16 energizes the control motor 28 of the involved wheel, causing the wheel-related piston valve component to slide upward into the upstream chamber 25. Figure 3 ),keep away Figure 2 The initial inactive position. The piston valve assembly 23 reaches a position with an axial offset towards the upstream chamber 25, where the free end of the tapered lip of the intermediate sealing gasket 33 is in a closed position, sealingly engaging with the smooth cylindrical surface 235 of the central section 233 of the piston valve assembly 23. At this position of the piston valve assembly ( Figure 3 The intermediate sealing gasket 33 fluidly isolates the downstream chamber 24 and the upstream chamber 25. Therefore, the fluid communication between the master cylinder and the brake caliper is temporarily interrupted.

[0039] This fluid isolation state is a transient condition because the electric motor 28 drives the piston valve component further into the upstream chamber 25 ( Figure 4 Because the diameter D2 of the downstream end section 232 of the piston valve component 23 is smaller than the diameter D3 of the central section 233. Figure 5 The piston valve component 23 is pushed into the upstream chamber 25 or further into the upstream chamber 25, causing the volume of the downstream chamber 24 to increase, thereby instantly releasing the brake fluid pressure connected to the brake caliper.

[0040] According to a pressure regulation operation mode, BBCU 16 controls electric motor 28 to cause piston valve component 23 to move back and forth repeatedly multiple times per second during braking, so that piston valve component 23 performs linear reciprocating motion between the following positions: - Figure 3 The fluid isolation state is achieved, wherein the intermediate sealing gasket 33 seals the downstream chamber 24 and the upstream chamber 25. - Figure 4 The position, in which the intermediate sealing gasket 33 still seals the downstream chamber 24 and the upstream chamber 25, but the piston valve component 23 further enters the upstream chamber 25, thereby increasing the volume of the downstream chamber 24 and reducing the pressure.

[0041] Therefore, the intermittent braking torque delivered to the brake calipers can prevent the wheels from locking up or rolling over.

[0042] It should be noted that during the pressure regulation operation mode, the downstream chamber 24 remains fluidly isolated or sealed from the upstream chamber 25, so the master cylinder cannot exert any effect on the brake caliper.

[0043] In pressure regulation mode, the BBCU controls the electric motor and piston valve assembly until the pressure in the downstream chamber is detected by the hydraulic pressure sensor to drop below a predetermined threshold. Then, it sends a control signal to the electric motor 28 to return the piston valve assembly 23 to its initial position. Figure 2 Switch back to the normal operating mode of the ABS valve.

[0044] Due to the design and placement of the intermediate sealing gasket 33, the fluid isolation or sealing function performed by the intermediate sealing gasket 33 can be maintained as long as the pressure in the upstream chamber 25 is greater than or equal to the pressure in the downstream chamber 24. Conversely, when the rider releases the brake lever and the pressure in the upstream chamber 25 is less than the pressure in the downstream chamber 24, the intermediate sealing gasket 33 allows brake fluid to return from the downstream chamber 24 to the upstream chamber 25. Even when the rider releases the brake lever, it prevents braking torque from being applied to the brake caliper.

[0045] An algorithm manages the complete axial movement of the piston until it reaches... Figure 3 The isolation operation mode shown in the image has the intermediate sealing gaskets arranged as follows: Figure 3 As shown, the brake caliper is fluidly isolated from the master cylinder. If this condition is still insufficient to effectively control braking, the algorithm will adjust the pressure in the downstream chamber according to the pressure regulation operation mode described above.

[0046] As an alternative to the aforementioned pressure regulation operation mode, valve unit 20 can be controlled in a more conventional operation mode. According to an alternative operation mode, upon detection of a critical (slippage or rollover) braking condition, the BBCU energizes the electric motor 28, causing the piston valve component 23 to... Figure 2initial position driven to Figure 4 The sealing or isolation position allows for the release of hydraulic circuit pressure between the intermediate sealing gasket 33 and the brake caliper. Once the BBCU detects via the hydraulic pressure sensor that the pressure in the downstream chamber has dropped below a predetermined threshold, the BBCU sends a control signal to the electric motor 28, causing the piston valve assembly 23 to return to its initial position. Figure 2 This reopens the brake fluid flow in channel 234 and temporarily re-establishes direct fluid communication between the master cylinder and the brake caliper. Upon detecting a sustained critical braking condition, the BBCU immediately drives the piston valve assembly back to... Figure 4 The process of opening and closing the channel 234 at the sealed or isolated position is repeated several times per second during braking to prevent the vehicle from skidding.

[0047] The arrangement of diameters D3>D2>D1 in the central section 233, downstream section 232, and upstream section 231 of the piston valve component 23 provides a favorable safety failure condition. When the piston 23 is in Figure 4 When the electric motor 28 is de-energized, although the intermediate sealing gasket 33 fluidly isolates the master cylinder from the brake caliper, the piston valve component 23 moves to the downstream chamber 24 (to the right) until the passage 234 reopens because the diameter D3 of the central section 233 is wider than the diameter D1 of the upstream end section 231.

[0048] Preferably, the diameter D2 of the downstream end section 232 is larger than the diameter D1 of the upstream end section 231. Therefore, once the passage 234 reopens, the piston valve member 23 will be effectively pushed towards the downstream chamber 24 (to the right) due to the presence of the same hydraulic pressure in both the downstream and upstream chambers. This is because the annular area defined by diameters D1 and D3 (on the upstream chamber side) is larger than the annular area defined by diameters D2 and D3 (on the downstream chamber side). Therefore, the hydraulic pressure acting on the opposite side of the piston valve member 23 will provide a net axial force, pushing it towards the downstream chamber 24 and further into the downstream chamber 24, achieving... Figure 2 The valve unit is positioned accordingly. Therefore, it always ensures sufficient braking torque to bring the vehicle to a stop within a reasonable stopping distance, even in the event of a power failure.

[0049] Although specific embodiments of the invention have been disclosed, it should be understood that these disclosures are for illustrative purposes only and the invention should not be limited in any way. Various modifications will be apparent to those skilled in the art based on the above examples. The scope of the invention is limited only by the scope of the appended claims.

Claims

1. A valve unit (20) for an anti-lock hydraulic brake system of a bicycle, said valve unit (20) comprising: an outer valve body (21) defining an internal cavity (22) having an axial elongated shape; a piston valve member (23) axially slidingly housed within said internal cavity (22) and dividing an upstream chamber (25) and a downstream chamber (24) within said internal cavity (22), said piston valve member (23) being connectable to an electric motor (28) for controlling the axial position of said piston valve member (23) along said internal cavity (22); an inlet port (27) formed in said outer valve body (21) for establishing fluid communication between said upstream chamber (25) and a brake operating hydraulic master cylinder (13); an outlet port (26) formed in said outer valve body (21) for establishing fluid communication between said downstream chamber (24) and a brake caliper (11) operatively associated with said valve unit (20), wherein said inlet port (27) and said outlet port (26) are axially spaced from each other along said internal cavity (22); said piston valve member (23) having a downstream end section (232) having a diameter (D2), a central section (233) having a smooth cylindrical surface and a diameter (D3), and one or more radially narrower surfaces (234) axially extending from said smooth cylindrical surface (235) towards an upstream end section (231), wherein said diameter (D3) of said central section (233) is greater than said diameter (D2) of said downstream end section (232); three stationary annular sealing gaskets (31, 32, 33) mounted on said outer valve body (21) within said internal cavity (22) and slidingly sealingly engaging said piston valve member (23): (i) an upstream end sealing gasket (31) engaging said upstream end section (231); (ii) a downstream end sealing gasket (32) engaging said downstream end section (232), wherein said upstream and downstream end sealing gaskets (31, 32) are axially spaced from each other so that said inlet port (27) and said outlet port (26) are axially located between said upstream and downstream end sealing gaskets (31, 32); and (iii) an intermediate sealing gasket (33) mounted within said internal cavity (22) between said inlet port (27) and said outlet port (26), wherein said piston valve member (23) is axially drivable between a normal braking position and at least one fluid isolation position. In the normal braking position, the smooth cylindrical surface (235) is axially offset from and does not engage the intermediate sealing washer (33), and the intermediate sealing washer (33) is laterally aligned with and radially separated from the one or more radially narrower surfaces (234), defining one or more respective passages (236) between the piston valve member (23) and the intermediate sealing washer (33), thereby establishing fluid communication between the downstream and upstream chambers (24, 25); and In the at least one fluid isolation position, the piston valve member (23) is axially offset towards the upstream chamber (25), and the intermediate sealing washer (33) engages the smooth cylindrical surface (235), thereby fluidly isolating the downstream chamber (24) from the upstream chamber (25).

2. The valve unit of claim 1, wherein the outer valve body (21) defines the internal cavity (22), the outer valve body (21) being made of a plastic material.

3. The valve unit of claim 1 or claim 2, wherein the upstream end section (231) of the piston valve member (23) has a diameter (D1) that is less than a diameter (D3) of the central section (233).

4. The valve unit of claim 3, wherein the diameter (D2) of the downstream end section (232) is greater than the diameter (D1) of the upstream end section (231).

5. The valve unit of any preceding claim, wherein the intermediate sealing washer (33) has a tapered lip that tapers towards the upstream chamber (25).

6. The valve unit of claim 1, wherein the one or more radially narrower surfaces (234) comprise slots that extend axially and are circumferentially spaced.

7. The valve unit of claim 1, wherein: the outer valve body (21) is shaped as a generally tubular body having axially opposite open ends and a radially thicker central wall portion (211) providing a lateral surface (212) facing the upstream chamber (25); an upstream hollow tubular insert (35) is fitted within the internal 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 washer (33) is stably held in an axial position between the central wall portion (211) and the upstream hollow tubular insert (35).

8. The valve unit of any preceding claim, wherein the valve unit is configured so that the piston valve member (23) can be driven to perform linear reciprocating motion and repeatedly and alternately reach two opposite axial displacement fluid isolation positions according to a pressure modulation operating mode: a first fluid isolation position in which the piston valve member (23) is axially offset towards the upstream chamber (25) and the intermediate sealing gasket (33) engages the smooth cylindrical surface (235) thereby establishing fluid isolation between the downstream chamber (24) and the upstream chamber (25); a second fluid isolation position in which the piston valve member (23) is further axially offset towards and into the upstream chamber (25) and the intermediate sealing gasket (33) engages the smooth cylindrical surface (235) thereby fluidly isolating the downstream chamber (24) from the upstream chamber (25).

9. The valve unit according to any one of claims 1 to 7, wherein the valve unit is configured such that the piston valve member (23) is drivable according to an operating mode to perform a linear reciprocating motion and repeatedly and alternately reach two relative axial displacement positions: the normal braking position in which the smooth cylindrical surface (235) is axially offset from the intermediate sealing gasket (33) to disengage the intermediate sealing gasket (33) and the intermediate sealing gasket (33) is laterally aligned with and radially separated from the one or more radially narrower surfaces (234) to define one or more respective passages (236) between the piston valve member (23) and the intermediate sealing gasket (33) to establish fluid communication between the downstream and upstream chambers (24, 25); and the at least one fluid isolation position in which the piston valve member (23) is axially offset towards the upstream chamber (25) and the intermediate sealing gasket (33) engages the smooth cylindrical surface (235) to fluidly isolate the downstream chamber (24) from the upstream chamber (25).

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

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