Cooling System Pressure Cap: Inspection
Hold the cap in your hand, top side up . (Fig 1). The vent valve at the bottom of the cap should open. If the rubber gasket has swollen, preventing the valve from opening, replace the cap.
Hold the cleaned cap in your hand, upside down. If any light can be seen between vent valve and the rubber gasket, replace the cap. DO NOT use a replacement cap that has a spring to hold the vent shut.
A replacement cap must be of the type designed for coolant reserve systems. This design ensures coolant return to the radiator.