Hydraulic master cylinder

The hydraulic cylinder for a hydraulic system, especially a master cylinder for a hydraulic clutch, has a housing (12) with a bush formed therein for a longitudinally movable piston (18) which determines a variable pressure chamber (32) which is sealed against atmospheric pressure by a primary seal (36) installed in the housing. A sensing system senses the position of the piston in the cylinder and comprises a permanent magnet (56) asymmetrically and rotationally fixed on the piston in a circumferential position aligning with a stationary sensor (61,63) which is sensitive to magnetic fields.