Unloader for a source of air under pressure on vehicles

A compressed air unloader for unloading compressed air used in the compressed air braking and suspension circuits of a motor vehicle is disclosed. The unloader includes a housing with an internal air chamber, an air inlet for passing compressed air into the air chamber, a compressed air outlet, and a controlled outlet valve in the compressed air outlet for venting compressed air in the air chamber to the atmosphere when the valve is opened. An electronic control unit opens and closes the outlet valve. The air chamber also has an air chamber check valve and a pressure sensor downstream of the check valve. An integral multi-circuit control valve assembly is formed within the housing, the control valve assembly including a plurality of generally identical control and monitor units positioned downstream of the air chamber check valve and the pressure sensor, each of the control and monitor units comprising a control and monitor unit check valve opening against the direction of the air flow therethrough, an overflow valve for passing compressed air from the control and monitor unit, an actuator for operating the overflow valve, and an air pressure sensor positioned downstream of the control and monitor unit check valve.