Radiator Cap: Description and Operation

Figure 5 Pressure Cap:

Pressure bottle is equipped with a pressure cap which release pressure at some point within a range of 117 kPa ± 7 kPa (17 psi ± 1 psi).
The system will operate at higher than atmospheric pressure which raises the coolant boiling point thus allowing increased radiator cooling capacity.
There is a spring-loaded vent valve in the center of the cap that allows the system to pressurize and Repressurize without creating a vacuum. If the valve is stuck open, coolant will escape to the overflow hose. There is also a gasket in the cap to seal to the too of the filler neck.