I rented out one of these to test my head gasket recently but have yet to use it. Sounds like they work well, but I was just curious on a part of the instructions. It states "With engine warm and idling" as you use the kit on the radiator hole. My question is how do you open up the radiator cap if you are on a warm engine, taking that as 'full operating temperature'? Though limited, my knowledge has always been you don't want to open the radiator cap if the engine is hot. Do you just do it very very slowly?