Patent ID: 8156740

Claim:
Brake master cylinder comprising, from a brake pedal to a bottom ( 2 ) of the master cylinder, a master cylinder body ( 3 ), said master cylinder body comprising a chamber ( 4 ) situated in a bore ( 5 ) of the body of the master cylinder, a piston ( 6 ) sliding in the bore on an axis ( 7 ) of displacement of the piston, a reservoir ( 8 ) of hydraulic liquid supplying the chamber via an orifice ( 14 ) in the piston and a supply duct ( 9 ) in the master cylinder body, a sealing cup ( 17 ), the sealing cup being in contact via an inner lip ( 19 ) with the periphery of the piston, and via an outer lip ( 20 ) with the body of the master cylinder, said orifice being either facing said inner lip or more distant, depending on the position in displacement of the piston on an axis of displacement of said piston, from the supply duct than the inner lip during a braking action, the piston comprises a frustoconical portion ( 24 ), the frustoconical portion of the piston is formed by an inclined wall ( 25 ) relative to an axis of displacement of the piston, characterized in that said inclined wall forms an incurved convex rounded surface ( 32 ) from an external diameter of the piston to a diameter ( 33 ) smaller than the external diameter of the piston, said rounded surface being such that an entrance ( 34 ) of the orifice is wholly situated in the rounded surface.