a) a pressure-loaded diaphragm, which is arranged in a housing of a service brake cylinder and which delimits, on one side, a service brake chamber acted upon with service brake pressure and, on the other side, a chamber which holds a restoring spring;

b) a spring brake piston, which is arranged in a housing of the spring-loaded brake cylinder and which is actuatable by a preloaded spring and which delimits, on one side, a spring brake chamber and, on the other side, a spring chamber which holds a preloaded spring and which has a piston rod;

c) an air-bleed valve, carried in the piston rod, which air-bleed valve produces or blocks a flow connection between the spring chamber and the service brake chamber;

wherein the air-bleed valve comprises:

(1) a piston which carries a valve body, which piston is guided in an axially movable manner in a cylinder, with at least one first pressure spring supported on the piston loading the valve body against a first valve seat on the piston;

(2) the piston is loaded by at least one second pressure spring, which is supported against the piston rod, such that the valve body is forced in a direction of a second valve seat on the piston rod and away from the first valve seat;

(3) a first piston face of the piston is loaded by pressure in the spring chamber in a direction which lifts the valve body from the second valve seat, and a second piston face is loaded by pressure in the service brake chamber in a direction which forces the valve body against the second valve seat and lifts said valve body up from the first valve seat; and

(4) a flow connection between the service brake chamber and the spring chamber is produced when the valve body is lifted up from the first valve seat and/or when the valve body is lifted up from the second valve seat.