The multi-functional APU (Air Processing Unit) combines several individual units. It includes an air dryer with or without a heater, a safety valve and a tire inflation connector. A multiple-circuit protection unit with one or two integrated pressure limited valves and two integrated check valves is flanged onto the air dryer. Some variants also have a double pressure sensor attached to the multiple-circuit protection valve for picking up the supply pressures of the service braking circuits. Compared with individual valves, WABCO's APU offers the benefit of being a compact unit which is not only cheaper and easier to install, but which also requires less space for installation.