This master cylinder comprises a body (12) defining a primary pressure chamber (14) and a secondary pressure chamber (16), separated by an axially movable piston (22), the so-called secondary piston; means (28) for the hydraulic connection of the primary pressure chamber (14) with braking-force simulation means (30); and complementary moving and stationary means (32, 34) for the sealing of the hydraulic connection means (28). The moving and stationary sealing means (32, 34) are borne by the secondary piston (22) and the body (12) respectively. The secondary piston (22) is axially movable between a rest position, in which the complementary sealing means (32, 34) are spaced apart from each other, and a position in which said complementary sealing means (32, 34) are cooperating with each other. The secondary piston (22) comprises two portions (22P, 22D), capable of an axial motion in relation to each other in order to adjust the relative axial position of the moving and stationary sealing means (32, 34) when the secondary piston (22) is in its rest position.