Abstract: A master cylinder having a body (1) pierced with a bore (100) that is partially closed by a guide ring (11). A piston (3) mounted to slide in the guide ring (11) and bore (100) to define a pressure chamber (4) within the bore (100). An inlet (2) having a filling duct (21) which connects the bore (100) with a source of hydraulic fluid having a low pressure when the piston (3) is in a position of rest. The filling duct (21) is located between a flared portion (104) of the bore (100) and a front edge (62) of a ring (6) which slides in the body (1) between the guide ring (11) and pressure chamber (4). The piston (3) and sliding ring (6) have respective a first stop (31) and a second stop (61) which allows the piston (3) to drive the sliding ring (6) along when the piston returns to a position of rest.