Review on High Performance Stant 10231 Radiator Cap - Vented (16 PSI) by Ron Long

This stant cap is not a full pressure cap. This is "partial pressure". The vacuum relief valve will remain open until the coolant begins to boil. The whole point of a sealed lid in a cooling system is to prevent water from boiling in the first place. This cap won't do it because it won't pressurize the cooling system.