Pressure Relief Valve
WARNING: This page is about a different car, the 1996 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.
Pressure relief valve, located in high pressure hose (near compressor), protects refrigerant system from excessive pressure. If high side pressure exceeds 450 psi (31.6 kg/cm2 ), valve opens, venting refrigerant to atmosphere. After excessive pressure is relieved, valve closes to prevent total loss of refrigerant.