High Pressure Relief Valve
WARNING: This page is about a different car, the 2010 Dodge Charger and 2010 Chrysler 300. However, it is still accessible from the selected car via links, so may be relevant.
A high pressure relief valve is located on the compressor cylinder head at the rear of the A/C compressor. See Heating and Air Conditioning/Plumbing/COMPRESSOR, A/C - Description . This mechanical valve is designed to vent refrigerant from the A/C system to protect against damage to the A/C compressor and other A/C system components caused by condenser air flow restriction or an overcharge of refrigerant.