Pressure Relief Valve
WARNING: This page is about a different car, the 1995 Ford Pickup, 1995 Ford F-Super Duty, 1995 Ford Cab & Chassis, and 1995 Ford Bronco. However, it is still accessible from the selected car via links, so may be relevant.
A pressure relief valve is installed on compressor to protect system from excessive pressure. Relief valve will shut off power to magnetic clutch when system pressure exceeds 450 psi (31.6 kg/cm2 ).