Answer: Most air compressors have a one way check valve between the compressor pump and the air receiver. When the compressor shuts off the check valve prevents the high pressure air from traveling back from the receiver to the pump. This allows for an unloader valve to release the pressure on the pump to atmosphere to allow for easier startup. The unloader system can be of several different designs. Two of the more common types are pressure switch mounted or centrifugal mounted on the end of the crankshaft.