CN109154314B

CN109154314B - Hydraulic master cylinder with improved sealing device

Info

Publication number: CN109154314B
Application number: CN201780009881.5A
Authority: CN
Inventors: 迭戈·罗西; 艾里斯修·普拉托; 克里斯托夫·莫里尔; 山德罗·博纳尔多; 弗拉维奥-艾里斯修·福尼利亚
Original assignee: Valeo Embrayages SAS
Current assignee: Valeo Embrayages SAS
Priority date: 2016-01-15
Filing date: 2017-01-12
Publication date: 2020-02-28
Anticipated expiration: 2037-01-12
Also published as: FR3046820B1; CN109154314A; FR3046820A1; WO2017121966A1

Abstract

The invention relates to a hydraulic master cylinder (1) comprising: a main body (2) provided with an inner hole (3) and a piston (4) capable of sliding in the inner hole; a sealing device (12) comprising a primary lip (17) and a secondary lip (18) axially interposed between the primary lip (17) and the supply pipe (10); wherein the secondary lip (18, 118) forms an elastically deformable valve and is arranged such that over an initial portion of the piston stroke towards its fluid discharge position, the secondary lip (18) provides sealing contact against the contact surface when the pressure within the hydraulic chamber is greater than or equal to a threshold value, and deforms so as to allow fluid to flow into the supply pipe (10) and towards the hydraulic chamber (6) when the pressure within the hydraulic chamber falls below the pressure threshold value.

Description

Hydraulic master cylinder with improved sealing device

Technical Field

The present invention relates to the field of hydraulic control systems.

And more particularly to hydraulic master cylinders, such as those used in control systems for motor vehicle clutches.

Background

Hydraulic systems are known that make it possible to control a motor vehicle clutch. Such a hydraulic system comprises: a master cylinder associated with a clutch pedal of a vehicle; a slave cylinder associated with a stop capable of acting on the clutch to move it between its engaged and disengaged positions; and a hydraulic circuit that allows fluid to circulate between the slave cylinder and the master cylinder.

A hydraulic master cylinder for a clutch control system is described in document FR 2,924,185. The master cylinder comprises a body provided with an internal bore in which a piston slides axially, defining, with the internal bore, a hydraulic chamber of variable volume. During the disengagement phase, i.e. when the driver presses the clutch pedal, the piston moves within the internal bore from the idle position towards a discharge position discharging the fluid towards the master cylinder. Conversely, when the clutch pedal is released and the clutch returns to its engaged position, the piston moves from its exhaust position to its idle position.

The body has a supply pipe intended to be connected to a hydraulic fluid reservoir and emerging radially inside the inner bore behind the hydraulic chamber. The master cylinder comprises sealing means interposed radially between the body and the piston. The sealing tool includes a rear seal that permanently provides a seal behind the bore and the front seal. A front seal is mounted in the housing of the bore and has a lip that engages the outer cylindrical surface of the piston. The front end of the piston includes an axial groove formed in the thickness of the piston, and an opening is formed between the lip of the seal and the outer circumferential surface of the piston through which fluid circulates when the piston is in its idle position. Thus, when the piston is in its idle position, the front seal allows fluid to circulate between the hydraulic chamber and the supply pipe, so as to allow a fluid resupply of the hydraulic chamber. Conversely, the front seal provides a seal between the body and the piston when the piston is out of its idle position, thereby preventing fluid flow between the supply tube and the hydraulic chamber.

By analyzing the dynamic characteristics of such hydraulic master cylinders, hysteresis has been observed between the outlet stroke of the piston (by the disengagement phase of the pedal) and its return stroke (by the engagement phase of the release pedal), in particular due to the presence of a spring that returns the piston to its idle position. This phenomenon creates a relatively low pressure in the hydraulic chamber during the final part of the return stroke of the piston, which may promote the entry of air into the hydraulic chamber. However, such air ingress should be prohibited because it can cause the stroke of the clutch bearing to change, which can disrupt operation of the clutch control system.

It is of course known that the dimensions of the front seal are such that its lip is elastically deformed in order to allow the circulation of fluid from the supply pipe towards the hydraulic chamber of the master cylinder when the relative low pressure in the hydraulic chamber exceeds a threshold value. However, it is not possible to dimension the front seal such that, on the one hand, it is sensitive to low pressure levels in the hydraulic chamber and, on the other hand, it reliably ensures sealing of the hydraulic chamber for positive pressures of the pressure levels involved during the stroke of the piston towards its discharge position.

Therefore, a master cylinder of the above type is not entirely satisfactory.

Disclosure of Invention

One idea underlying the invention is therefore to propose a reliable hydraulic master cylinder.

According to one embodiment, the present invention provides a hydraulic master cylinder comprising a body having an internal bore and a piston slidable within the internal bore and defining with the internal bore a variable volume hydraulic chamber; the piston is movable within the bore between an idle position and a discharge position discharging fluid outside the hydraulic chamber;

the body comprises a fluid discharge tube present at one end of the hydraulic chamber and intended to be connected to the slave cylinder and to a fluid supply tube present in a portion of said internal bore adjacent to said hydraulic chamber;

the hydraulic master cylinder further comprises a sealing means arranged radially between the piston and the internal bore and axially between the hydraulic chamber and the supply pipe; the sealing means comprise a primary lip and a secondary lip axially interposed between the primary lip and the supply pipe, the primary lip and the secondary lip being supported on the one hand by one element from the body and the piston and, on the other hand, being in dynamic cooperation with a contact surface arranged on the other element from the body and the piston; the contact surface is arranged such that:

in the idle position, the primary and secondary lips do not provide sealing contact against the contact surface, so as to allow circulation of fluid from the supply pipe towards the hydraulic chamber;

in the initial part of the stroke of the piston from its idle position towards its discharge position, only the secondary lip is able to provide sealing contact against the contact surface; and

in the final part of the stroke of the piston adjacent to the initial part, the main lip provides a sealing contact against the contact surface, so as to prevent the circulation of fluid between the supply pipe and the hydraulic chamber;

the secondary lip forms an elastically deformable valve and is arranged such that, in an initial portion of the piston stroke, the secondary lip provides sealing contact against the contact surface when the pressure within the hydraulic chamber is greater than or equal to a pressure threshold, and the secondary lip deforms to allow fluid to circulate from the supply pipe towards the hydraulic chamber when the pressure within the hydraulic chamber falls below said pressure threshold.

The sealing means thus comprise two dynamic lips, each adapted to a specific function. The primary lip can withstand considerable working pressures and therefore a seal between the supply pipe and the hydraulic chamber can be reliably ensured in the final part of the piston stroke, i.e. in the part where the pressure in the hydraulic chamber is the greatest. The secondary lip in turn has a low pressure valve function, so that when the piston approaches its idle position during its return stroke, negative relative pressure is prevented from occurring in the hydraulic chamber. Furthermore, thanks to the above arrangement, the secondary lip operates only in the initial part of the piston stroke, in which the pressure is low, which makes it possible to protect the secondary lip, since it is never subjected to considerable pressure.

This arrangement is therefore particularly reliable, since on the one hand the secondary lip can avoid negative relative pressures in the hydraulic chamber that could force air into the hydraulic chamber, and on the other hand the primary lip is able to perform its sealing function over the entire range of positive pressures that may occur in the hydraulic chamber.

According to other advantageous embodiments, such a hydraulic master cylinder may have one or more of the following features:

-the pressure threshold is lower than atmospheric pressure.

The pressure threshold corresponds to a relatively low pressure of between 0.05 bar and 0.4 bar.

One end of the secondary lip, which is oriented towards the hydraulic chamber, is in dynamic engagement with the contact surface.

The contact surface is arranged so that the initial portion of the piston stroke has a length comprised between 1mm and 5 mm.

The contact surfaces are arranged such that in an initial part of the piston stroke the relative pressure in the hydraulic chamber remains below 5 bar.

During the movement of the piston from its idle position to its expulsion position, the contact surface has, in succession, a first region, a second region, a third region and a fourth region in the direction of movement of the sealing device relative to the contact surface; the first and second regions cooperate with the secondary and primary lips, respectively, when the piston is in its idle position, and are each arranged to cooperate in a fluid-permeable manner with said secondary or primary lip; the third and fourth regions are capable of providing sealing contact with the secondary and primary lips, respectively; the first, second and third regions being arranged such that in an initial portion of the piston stroke, the secondary lip engages the second region and the primary lip engages the third region; and the primary lip engages the fourth region during the final portion of the piston stroke.

In other words, the first, second and third regions are arranged such that the secondary lip reaches the second region before the primary lip reaches the fourth region, and the secondary lip reaches the third region only after the primary lip reaches the fourth region.

The first zone and the third zone each have one or more axial slots to allow circulation of the fluid.

The primary and secondary lips are fixedly mounted on the piston and the contact surface is arranged on the cylindrical surface of the inner bore.

The main lip has an anti-friction coating.

The anti-friction coating is made of a material chosen from polytetrafluoroethylene and polyamides, such as polyamide 12.

According to a first embodiment, the primary lip and the secondary lip are supported by the primary seal and the secondary seal, respectively.

The primary seal is made of ethylene-propylene-diene monomer.

The primary seal comprises a core and an anti-friction coating overmoulded on the core and covering the primary lip.

The secondary seal is made of a silicone-based elastomer.

According to a second embodiment, the primary lip and the secondary lip are arranged in a single seal.

The single seal is made of a silicone-based elastomer.

The single seal comprises a core and an anti-friction coating overmoulded on the core and covering the primary lip.

The stroke of the piston between its idle position and the discharge position comprises, in turn, an initial portion and a final portion.

According to one embodiment, the invention also provides a motor vehicle comprising the aforementioned hydraulic master cylinder.

Drawings

The present invention will be better understood and other objects, details, features and advantages will become more clearly apparent in the following description of several specific embodiments thereof, given by way of illustration and not of limitation, with reference to the accompanying drawings.

Fig. 1 is a partial axial section of a hydraulic master cylinder.

Figures 2 to 5 are detailed cross-sectional views of successive stages of the sealing device according to the first embodiment during the stroke of the piston from its idle position towards its fluid discharge position.

Figures 6 to 9 are detailed cross-sectional views of successive stages of the sealing device according to the second embodiment during the travel of the piston from its idle position to its fluid discharge position.

Detailed Description

In the description and in the claims, the terms "external" and "internal" and the orientations of "axial" and "radial" will be used to indicate the elements of the hydraulic master cylinder according to the definitions given in the description. By convention, the "radial" orientation is orthogonal to the sliding axis X of the piston of the hydraulic master cylinder determining the "axial" orientation and moves away from said axis from the inside towards the outside. The terms "outer" and "inner" are used to define the relative position of one element with respect to the other with respect to the axis X; thus, the elements close to the axis are described as being inner, as compared to the outer elements which are radially on the periphery. Furthermore, the terms "rear" AR and "front" AV are used to define the relative position of one element with respect to another element in the axial direction.

Fig. 1 partially shows a hydraulic master cylinder 1. The hydraulic master cylinder 1 comprises a body 2 of unitary tubular shape extending axially along an axis X. The body 2 may in particular be made of plastic, for example polyamide 6-6 reinforced with glass fibres.

The body 2 comprises an internal bore 3, in which bore 3 a piston 4 slides along an axis X. The piston 4 is connected to a control rod (not shown) which in turn is attached to an actuating means such as a clutch pedal.

The front portion of the body 2 is axially defined by a transversely oriented bottom 5, while the rear portion (not shown) can be closed by a cover attached at the rear end of the body 2. The body 2 has in front a variable volume hydraulic chamber 6 axially delimited from front to rear by a bottom 5, a cylindrical surface of the internal bore 3 and the front of the piston 4.

The main body 2 comprises at the front a coupling 7 communicating with the hydraulic chamber 6 through a discharge duct 8 for the fluid passing through the bottom 5 of the main body. The coupling 7 is for receiving the end of a tube connected to a slave cylinder (not shown). Thus, the hydraulic master cylinder 1 can discharge the fluid contained in the hydraulic chamber 6 toward the master cylinder. The fluid is, for example, brake fluid or oil, such as transmission oil.

Furthermore, the hydraulic master cylinder 1 comprises a coupling 9 intended to be connected to a reservoir (not shown) for supplying fluid to the hydraulic master cylinder. The coupling 9 communicates with the interior of the bore 3 via a fluid supply tube 10, the fluid supply tube 10 radially traversing the cylindrical wall of the bore 3 and emerging in a section of the bore 3 located behind the hydraulic chamber 6.

The piston 4 is movable between a rear idle position shown in fig. 1 and a front fluid-discharge position, not shown, in which the piston 4 drives fluid towards the hydraulic follower in order to move the clutch towards its disengaged position. Thus, when the driver disengages, the control rod moves forward, so that the piston 4 moves from its idle position to its fluid-discharge position from the rear to the front, disengaging the clutch.

The piston 4 is made of plastic or thermoplastic material, and preferably thermoplastic material.

The hydraulic master cylinder 1 may comprise means for returning the piston 4, which are able to return the piston 4 to its idle position. The return means are constituted, for example, by a spring, not shown, housed in the hydraulic chamber 6 and acting between the bottom 5 of the body 2 and the front end of the piston 4.

The hydraulic master cylinder 1 comprises, towards the rear, a dynamic sealing gasket 11 for providing a constant seal between the supply pipe 10 and the rear of the body 2. In the embodiment shown, a dynamic sealing gasket 11 is fitted around the piston 2 and axially fastened to the piston 2. The dynamic sealing gasket 11 comprises a radial lip cooperating with the cylindrical surface of the internal bore 3 in order to guarantee sealing. In another alternative embodiment, not shown, the dynamic sealing gasket 11 is mounted in a complementary housing arranged in the cylindrical surface of the inner bore 3 and comprises a radial lip cooperating with the outer circumferential surface of the piston 2.

The hydraulic master cylinder 1 further comprises a sealing device 12 located at the front. This sealing device is shown in detail in fig. 2 to 5.

The sealing means 12 are interposed radially between the body 2 and the piston 4 on the one hand and axially between the hydraulic chamber 6 and the supply pipe 10 on the other hand. As will be explained below, the sealing means 12 are arranged, on the one hand, to allow circulation of fluid from the supply pipe 10 towards the hydraulic chamber 6 when the piston 4 is in its idle position, in order to allow resupply of liquid to the hydraulic chamber 6, and, on the other hand, to prevent circulation of fluid between the supply pipe 10 and the hydraulic chamber 6 when the piston 4 is moved from its idle position towards its fluid discharge position.

The sealing device 12 comprises two dynamic seals, a primary seal 13 and a secondary seal 14. The secondary seal 14 is arranged behind the primary seal 13 and is therefore inserted axially between the primary seal 13 and the supply pipe 10.

Each of the primary seal 13 and the secondary seal 14 is fitted on the piston 14 and is retained axially towards the rear by

respective shoulders

15, 16 arranged on the outer circumferential surface of the piston 4. The primary seal 13 and the secondary seal 14 comprise a primary lip 17 and a secondary lip 18, respectively, each of which dynamically cooperates with a contact surface arranged on the cylindrical surface of the inner bore 3.

In the embodiment of fig. 1-5, the primary seal 13 is a two-component seal. Indeed, as particularly shown in fig. 2 to 4, the primary seal 13 comprises a core 19 made of elastomeric material and a coating with a low friction coefficient 20 overmoulded on the core 19 and covering the primary lip 17. The core 19 of the primary seal 13 is made, for example, of ethylene-propylene-diene monomer, generally indicated by the abbreviation EPDM. The coating with the low coefficient of friction 20 is in turn made of a material selected from the group consisting of polytetrafluoroethylene and polyamide 12. Coatings with a low coefficient of friction 20 attempt to limit the noise generated by the main seal 13 rubbing against the main body 2. The presence of a coating having a low coefficient of friction 20 is particularly advantageous for the main seal 13, since the elastomeric materials that can be used to make the seal gasket generally have a high coefficient of friction when the seal is required to withstand high pressures, which may generate frictional noise.

The secondary lip 18 is not as hard as the primary seal and forms a valve that is deformable when subjected to pressures on the hydraulic chamber side 6 below a pressure threshold. For this purpose, the end of the secondary lip 18 in contact with the body 2 is oriented towards the hydraulic chamber 6.

The pressure threshold is lower than atmospheric pressure and advantageously corresponds to a relative low pressure of between 0.05 bar and 0.4 bar, and preferably of about 0.1 bar, which makes it possible to reasonably limit the level of relative low pressure that can be reached inside the hydraulic chamber 6. The secondary lip 18 has a flexibility and a length that are suitable and predetermined according to the nature of the assembly material and the pressure threshold below which it must be deformed. The secondary seal 14 is made of, for example, a silicon-based elastomeric material.

As shown in fig. 2, the contact surface of the internal bore 3 that cooperates against the primary and

secondary lips

17, 18 comprises, from front to rear, a first region 21 and a third region 23 cooperating with the secondary and

primary lips

18, 17, respectively, when the piston 4 is in its idle position. In each of the first and

third regions

21, 23, one or more

axial grooves

25, 26 are arranged. The

axial grooves

25, 26 form openings between the

secondary lips

18 and 17 and the inner bore 3, through which fluid circulates, so that the hydraulic chamber 6 can be supplied again.

The first and

third regions

21, 23 are axially separated from each other by a second region 22, which is free from axial grooves, while the third region 23 extends in the forward direction of a fourth region 24, which is also free from axial grooves. As shown in fig. 4, the secondary lip 18 can provide sealing contact against the third region 23, while the primary lip 17 can provide sealing contact against the fourth region 24.

In the embodiment shown, the first and

second regions

21, 22 are arranged on a diameter of the inner bore 3, which diameter of the inner bore 3 is larger than the diameter of the inner bore measured at the third and

fourth regions

23, 24. The second region 22 and the third region 23 are connected to each other by a frustoconical portion.

The relative lengths of the first, second and

third regions

21, 22, 23 are such that during the movement of the piston 4 from its idle position to its fluid-discharge position:

before the primary lip 17 reaches the fourth region 24, the secondary lip 18 reaches the second region 22, which is able to provide sealing contact against the second region 22 (see fig. 3); and

the secondary lip 18 reaches the third region 23 only after the primary lip 17 has reached the fourth region 24, against which the primary lip 17 provides sealing contact (see fig. 4).

Thus, as shown in fig. 3, only the secondary lip 18 is able to provide sealing contact against the inner bore 3 during the initial part of the stroke of the piston 4 from its idle position towards its discharge position. In fact, during the initial part of the piston stroke, the secondary lip 18 cooperates with the second zone 22, which has no axial grooves, while the primary lip 17 cooperates with the third zone 23, which is equipped with one or more axial grooves 26.

As shown in fig. 4 and 5, in the remaining part of the stroke of the piston 3, hereinafter referred to as the final part, the seal between the supply pipe 10 and the hydraulic chamber 6 is provided at least by the primary lip 17, the primary lip 1 being in sealing contact against the fourth region 3 which is free from axial grooves.

During the return stroke of the piston 4 from its fluid discharge position towards its idle position, the piston 4 passes through the initial part of travel again, as shown in fig. 3, and then returns to its idle position, as shown in fig. 2. If, in the initial position of this stroke, the pressure in the hydraulic chamber 6 drops back below the pressure threshold at which the secondary lip 18 deforms, the secondary lip 18 allows fluid to circulate from the supply pipe 10 towards the hydraulic chamber 6 in order to rebalance the pressure. Due to such an arrangement, the hydraulic chamber 6 is prevented from being subjected to an excessive low pressure level.

Fig. 6 to 9 show a sealing device according to a second embodiment. Elements that are the same or similar to elements of fig. 1-5, i.e., perform the same function, have the same reference number, increased by 100.

The operation of the sealing device 112 is similar to that described above with respect to fig. 1-5, and the sealing device 112 differs from the preceding sealing device 12 only by its structure.

In this embodiment, the primary lip 117 and the secondary lip 118 are formed in the same seal 127. The seal 127 comprises a core 128 made of an elastomeric material, for example a silicone-based elastomer, and an anti-friction coating 129 covering the main lip. An anti-friction coating 129 is, for example, overmolded on the core 128 of the seal 127. The core 128 is made of an elastic material, such as a silicone-based elastomer. The anti-friction coating 129 is made of a material selected from, for example, polytetrafluoroethylene and polyamide 12.

Although the invention has been described in connection with several specific embodiments, the invention is of course not limited thereto and encompasses all technical equivalents of the means described as well as combinations thereof if they fall within the scope of the invention.

In particular, in other embodiments, not shown, the primary and secondary lips are stationary with respect to the body of the hydraulic master cylinder and comprise a radial lip which dynamically cooperates with a contact surface arranged on the outer cylindrical surface of the piston. In this case, the primary and secondary seals are each mounted in a complementary housing arranged in the cylindrical surface of the bore if the two lips are formed on separate seals, or the seal carrying the two lips is mounted in a complementary housing arranged in the cylindrical surface of the bore if the two lips are formed on the same seal. Furthermore, the contact surface arranged on the cylindrical surface comprises a series of four zones, similar to those previously described, the first, second, third and fourth zones being successive to one another from the front to the rear of the piston.

Use of the verb "to comprise", "comprise" or "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The use of the indefinite article "a" or "an" for an element or step does not exclude the presence of a plurality of such elements or steps, unless otherwise indicated.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Claims

1. A hydraulic master cylinder (1) comprising: a body (2, 102) provided with an inner bore (3, 103); and a piston (4, 104) slidable within the bore and defining with the bore a variable volume hydraulic chamber (6, 106); -the piston (4, 104) is movable within the bore (3, 103) between an idle position and a discharge position discharging fluid outside the hydraulic chamber (6, 106);

the main body (2, 102) comprises a fluid discharge pipe (8) present at one end of the hydraulic chamber (6, 106) and intended to be connected to a slave cylinder and a fluid supply pipe (10) present in a section of the internal bore (3, 103) adjacent to the hydraulic chamber (6, 106);

the hydraulic master cylinder (1) further comprises sealing means (12, 112) arranged radially between the piston (4, 104) and the internal bore (3, 103) and axially between the hydraulic chamber (6, 106) and the supply pipe (10, 110); the sealing means (12, 112) comprising a primary lip (17, 117) and a secondary lip (18, 118) axially interposed between the primary lip (17, 117) and the supply pipe (10, 110); -said primary lip (17, 117) and said secondary lip (18, 118) are supported on the one hand by one element from said main body (2, 102) and said piston (4, 104) and on the other hand dynamically cooperate with a contact surface arranged on the other element; the contact surface is arranged such that:

-in the idle position, the primary lip (17, 117) and the secondary lip (18, 118) do not provide sealing contact against the contact surface, so as to allow circulation of fluid from the supply pipe (10, 110) towards the hydraulic chamber (6, 106);

-in an initial portion of the stroke of the piston (4, 104) from its idle position towards its discharge position, only the secondary lip (18, 118) is able to provide a sealing contact against the contact surface; and

-in a final portion of the stroke of the piston (4, 104) adjacent to the initial portion, the primary lip (17, 117) provides a sealing contact against the contact surface, so as to prevent fluid from circulating between the supply pipe (10, 110) and the hydraulic chamber (6, 106);

the secondary lip (18, 118) forms an elastically deformable valve and is arranged such that, in the initial part of the stroke of the piston, the secondary lip (18, 118) provides sealing contact against the contact surface when the pressure within the hydraulic chamber is greater than or equal to a pressure threshold, and deforms to allow circulation of fluid from the supply pipe (10, 110) towards the hydraulic chamber (6, 106) when the pressure within the hydraulic chamber falls below the pressure threshold.

2. The hydraulic master cylinder (1) according to claim 1, wherein the pressure threshold is lower than atmospheric pressure.

3. Hydraulic master cylinder (1) according to claim 2, wherein said pressure threshold value corresponds to a relatively low pressure of between 0.05 bar and 0.4 bar.

4. A hydraulic master cylinder (1) according to any one of claims 1 to 3, wherein one end of the secondary lip (18, 118) dynamically cooperates with the contact surface, this end being oriented towards the hydraulic chamber.

5. A hydraulic master cylinder (1) according to any one of claims 1 to 3, wherein, during the movement of the piston from its idle position towards its discharge position, the contact surface has, in succession, a first region (21, 121), a second region (22, 122), a third region (23, 123) and a fourth region (24, 124) in the direction of movement of the sealing device (12, 112) relative to the contact surface; -when the piston (4, 104) is in its idle position, the first region (21, 121) and the second region (22, 122) cooperate with the secondary lip (18, 118) and the primary lip (17, 117), respectively, and are each arranged to cooperate in a fluid-permeable manner with the secondary lip (18, 118) or with the primary lip (17, 117); the third region (23, 123) and the fourth region (24, 124) being capable of providing sealing contact with the secondary lip (18, 118) and the primary lip (17, 117), respectively; the first, second and third regions being arranged such that, in the initial portion of the piston's stroke, the secondary lip (18, 118) engages with the second region (22, 122) and the primary lip (17, 117) engages with the third region (23, 123); and in the final portion of the piston stroke, the primary lip (17, 117) cooperates with the fourth region (24, 124).

6. The hydraulic master cylinder (1) according to claim 5, wherein the first and third regions (21, 121, 23, 123) each have one or more axial grooves (25, 26) to allow fluid circulation.

7. Hydraulic master cylinder (1) according to one of claims 1 to 3, wherein the primary lip (17, 117) and the secondary lip (18, 118) are fixedly mounted on the piston (4, 104), and wherein the contact surface is arranged on a cylindrical surface of the bore (3, 103).

8. The hydraulic master cylinder (1) according to any one of claims 1 to 3, wherein the primary lip (17, 117) has an anti-friction coating (20, 120).

9. Hydraulic master cylinder (1) according to claim 8, wherein the anti-friction coating (20, 120) is made of a material selected from polytetrafluoroethylene and polyamide.

10. Hydraulic master cylinder (1) according to any one of claims 1 to 3, wherein the primary lip (17) and the secondary lip (18) are supported by a primary seal (13) and a secondary seal (14), respectively.

11. The hydraulic master cylinder (1) according to claim 10, wherein the primary seal (13) is made of ethylene-propylene-diene monomer.

12. The hydraulic master cylinder (1) according to claim 10, wherein the secondary seal (14) is made of a silicone-based elastomer.

13. The hydraulic master cylinder (1) according to any one of claims 1 to 3, wherein the primary lip (117) and the secondary lip (118) are arranged in a single seal (127).

14. The hydraulic master cylinder (1) according to claim 13, wherein said single seal (127) is made of silicone-based elastomer.