The invention concerns a master cylinder comprising a body (12) delimiting a braking fluid supply chamber (14) and at least a primary pressurizing chamber (16), a primary piston (18) separating the supply and the primary chambers, axially mobile between an inactive position and a position for pressurizing the primary chamber (14), a feeding channel (40) and means (42) for connecting said feeding channel with the supply chamber. Said connecting means (42) include two axially offset openings (44, 46), called front and rear relative to the displacement direction of the primary piston (18) during braking, the front opening (46) capable of being at least partially closed, by displacement of the primary piston (18) such that, when the primary piston (18) is in its inactive position, the cross-section of the fluid passage through said front opening (46) is minimal.