Operation - Gas Engine
| 1 - NECK SEAL |
| 2 - VACUUM VENT |
| 3 - PRESSURE RATING |
| 4 - PRESSURE VALVE |
The pressure cap allows the cooling system to operate at higher than atmospheric pressure. The higher pressure raises the coolant boiling point; this allows increased radiator cooling capacity.
The secondary gasket in the cap seals the filler neck and the primary gasket seals the cooling system so that vacuum can be maintained, allowing coolant to be drawn back into the radiator from the reserve container.
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 coolant in reserve/overflow tank to be drawn through connecting hose into radiator. If the vacuum valve is stuck shut, or overflow hose is kinked, radiator hoses will collapse on cool-down.