TANDEM MASTER CYLINDER PRIMARY PISTON

Primary piston (10) of a master cylinder composed of a body (11) bearing a skirt (12) whose front (12a) has an annular outer crown (15) with grooves over an axial length (A) while remaining covered by the pressure cup (40) when the primary piston (10) is in the active position. These grooves communicating with the peripheral supply groove (51) when the piston (10) is at rest and they are covered by the cup (40) when the piston (10) is in the active position. A groove (30) has an extra length (B) with respect to the length (A) of the grooves (20) so as to form a calibrated passage (35) when the primary piston (10) is in the active position. The calibrated passage (35) prevents pressure locking of the primary piston without creating a brake fluid leak towards the reservoir (60).