Abstract: The dual braking unit includes a cylinder with a diaphragm 4, 11-17) for controlling the service braking and a cylinder with a drive spring (3, 6-10) for controlling the parking and emergency braking. The cylinder with the spring includes a piston (6, 10) with an operating tube or tubular push rod (10) which extends sealingly through a hole (5) in a wall (2) separating the operating chambers (8; 12) of the cylinders. There is a pair of holes (35, 36; 35, 34) in the tube (10) for enabling communication between the chamber (7) with the spring (9) and the operating chamber (12) of the cylinder with the diaphragm when only the parking and emergency brake is applied. A cut-off valve device (40-43) is associated with the holes (35, 36; 34. 35) for preventing air from flowing from the operating chamber (12) of the cylinder with the diaphragm to the chamber (7) with the spring (9) when the service brakes are applied while the parking and emergency brake is applied.