Radiator Cap - Operation

RADIATOR PRESSURE CAP

A vent valve in the center of the cap will remain shut as long as the cooling system is pressurized. As the coolant cools, it contracts and creates a vacuum in cooling system. This causes the vacuum valve to open and the coolant in reserve/overflow tank to be drawn through the connecting hose into radiator. If the vacuum valve is stuck shut, or the overflow hose is kinked, the radiator hoses will collapse on cool-down.