Master cylinder device for a car

The device has a position sensor controlling a stop light ignition syrod. The sensor comprises a magnetic field detector (21) fixed on a body (18) of a master cylinder (2). A magnet (27) is integrated with a rod (24) connected to a primary piston (13) for sliding the piston in a bore (23) parallel to another bore (19) in which the piston slides. The former bore includes two cylindrical rotating parts (28, 34), where diameter (36) of one of the parts located near the detector is larger than diameter (30) of other part located near a sliding zone (14) of the piston in the body of the cylinder.