Master-cylinder for vehicles with a system for detecting the braking action, in particular the position of the piston

The cylinder has a sliding piston (4) within a boring (3) to make the volume of a pressure chamber (5) variable. A detection device (9) is fixed on a body (2) opposite to a passage of the piston at the level of the chamber to detect an action of a brake system. The device has a magnetic circuit opened and closed by a magnetic part (19). The circuit has polar parts (10, 11), a magnet (12), and a magneto sensitive unit (13).