Pressure Cap: A pressure cap is a special cap for the radiator. It holds a predetermined amount of pressure on the water in the cooling system which enables the water to run hotter without boiling. In this way, the engine can operate at a higher and a more efficient temperature without danger of overheating. The cap also has a vacuum valve that lets air into the cooling system if the pressure falls too low to prevent outside air pressure from causing a partial collapse of the radiator.

Pressure Gauge: A pressure gauge is an instrument used for measuring the pressure exerted by the contents on its container. The unit for pressure in the gauge is usually pounds per square inch(psi) above the atmospheric pressure.