It is important to wash the radiator cap with just soap and water, since using any solvent may harm the seal. Beware that small leaks you find when testing the cap with a pressure gauge may not mean there is a problem with the cooling system. Just because there is a slow leak during testing doesn't mean you should replace a pressure cap that does not show leakage in its past. To try, pour water into the tool, turn it over and make sure the cap is still secure. Put a small amount of water on the gasket and tighten the cap on either a model 7700 pressure tester or an equivalent, before turning on the pump until the gauge shows 104 kPa (15 psi). Pressure caps that don't hold 97 kPa (14 psi) or more should be replaced. If the cap does not retain pressure or vacuum once it's installed on the radiator, look at the radiator filler neck and check if the top gasket of the cap is damaged, dirty or distorted. Put the cap upright, with the logo toward you and check if the valve at the base opens up and if it's blocked by a swollen rubber gasket, then you should replace the cap. Besides, tip the lid over and if light shines through the gap between the vent valve and the rubber gasket, replace it. A new cap without a spring to keep the vent closed and created for a coolant reserve/overflow should be used, together with a completely sealed diaphragm spring and a rubber gasket that blocks off the radiator filler top.