Master cylinder comprising a resupply-promoting seal

Brake master cylinder (10) for a motor vehicle, of the type which comprises a substantially axial body (12) inside a bore (14) of which is slideably mounted at least one axial piston (16, 18) which is capable of being actuated by a driver of the vehicle between a rear rest position and a forward braking force application position, and which is returned elastically toward its rear rest position, of the type in which the bore (14) comprises two seals, front (30, 34) and rear (32, 36), which are interposed between the piston (16, 18) and the bore (14), the front seal (30, 34) delimiting, in the bore (14), a rear supply chamber (38, 42) and a front pressure chamber (40, 44), of the type in which the body (12) comprises a radial supply duct (46, 52) which connects an external hydraulic fluid reservoir to the rear supply chamber (38, 42) and which opens out between the two seals (30, 32, 34, 36), of the type in which the body (12) comprises a braking circuit supply hole (54, 56) which opens into the front pressure chamber (40, 44), of the type in which the piston comprises a bore (58, 60), open to the front, communicating on the one hand with the front pressure chamber (40, 44) and on the other hand with the periphery of said piston (16, 18) by way of at least one hole (62, 64) which, when the piston (16, 18) occupies its rear rest position, is arranged between the two seals (30, 32, 34, 36) so as to open communication between the front pressure chamber (40, 44) and the rear supply chamber (38, 42) and which, when the piston (16, 18) is moved axially forward toward its application position, is capable of passing beyond the front seal (30, 34) so as to isolate the front pressure chamber (40, 44) from the rear supply chamber (38, 42) and thus allow a braking pressure to be established in the front pressure chamber (40, 44), of the type in which at least each front seal (30, 34) is accommodated in a groove (70, 72) in the body of the master cylinder and comprises three concentric lips with an axial orientation, in particular:

• a first inner lip (80) of which a free end (82) is arranged in contact with the periphery of the piston (16, 18), a second central lip (84) of which the free end (86) is arranged, in a rest position, in contact with a front face (88) of the groove (70, 72), and a third outer lip (80) of which a free end (92) is arranged, in a rest position, in contact with a peripheral bottom face (94) of the groove (70, 72), the second central lip (84) and the third outer lip (90) being capable, when the front pressure chamber (40, 44) is exposed to a partial vacuum caused by the return of the piston (16, 18) from its forward application position to its rest position or else by the activation of a trajectory control device forming part of the braking circuit, of separating from the front face (88) and from the peripheral bottom face (94) of the groove (70, 72) so as to allow the front pressure chamber (40, 44) to be resupplied by the reservoir, characterized in that the front face (88) of the groove is configured at least in part as a slope of defined inclination (a) so as to facilitate the separation by tilting of the second central lip (84), in order to facilitate the separation by tilting of the third outer lip (90).