a brake cylinder having a parking brake function includes a cylinder housing , a piston , and a piston tube for actuating a brake . a threaded bolt protruding into the piston tube and fixed to the cylinder housing by its upper end is used to effect the parking brake function . a clamping nut displaceable by means of a forked , axially rotatable driving element extends over the threaded bolt . the driving element is rotated with the clamping nut while the cylinder is subjected to pressure by the parking brake until the clamping nut is supported against the piston in the parking position of the piston . the piston is mechanically locked when the cylinder housing is not subjected to pressure .