Radiator Cap
Remove the cap. Be sure that the sealing surfaces are clean. Moisten the rubber gasket with water and install the cap on a commercially available pressure tester.
Using the cooling system tester, bring the pressure up to the pressure prescribed on cap. If the pressure cap fails to hold up to 20 psi, replace the cap.
The cap is designed to hold up to 20 psi. If during the test, the pressure rapidly bleeds off prior to 20 psi, the cap has failed.
The pressure cap may test properly while positioned on the pressure test tool. It may not hold pressure or vacuum when installed on the pressurized coolant bottle. If the test passes on the tool but there is sufficient evidence that the cap fails on the bottle, inspect the coolant bottle neck for damage or debris that may prevent the cap from sealing properly.
Radiator pressure testing tools are very sensitive to small air leaks which will not cause cooling system problems. A pressure cap that does not have a history of coolant loss should not be replaced just because it leaks slowly when tested with this tool. Add water to tool. Turn tool upside down and recheck pressure cap to confirm that cap needs replacement.