Master cylinder with increased safety and piston for such a master cylinder

A master cylinder comprising a body (2) in which is made a bore (3) blocked off at one end by an end wall (5), at least one piston (8, 10) slideably mounted in a sealed manner in the bore, a hydraulic fluid supply chamber formed in a longitudinal slot (30) in the piston, a working chamber connected to brakes, said working chamber being connected to the supply chamber through a passage (36) made in the piston (8, 10), a valve for blocking off said passage (36) during a braking phase, said valve (18) comprising a shutter (22) and a valve seat (20) borne by the piston (8, 10), said shutter (22) bearing at rest against a bearing element (42) fixedly mounted in the body (3) of the master cylinder, said bearing element being mounted to pass through the longitudinal slot (30), a distance (L) separating a first end (44) of the slot remote from the valve (20) and a first end (46) of the bearing element remote from the end wall (5) being less than a distance (D) separating a first end (34) of the piston close to the end wall (5) and the end wall (5), the bearing element being a pin, characterized in that the first end (44) of the slot (30) is in the form of a circular arc and in that the circular arc has a diameter which is substantially equal to the diameter of the pin so as to achieve uniform distribution of the stresses applied by the pin to the piston (8, 10).