Has anyone experienced the cooling system building up pressure all on its own? I'm useing a "Water Wetter" type of additive called "Cool It" and distilled water. When I first started the engine for break-in, the water became stained but since then it has cleared up nicely. After running the car, the last time, I noticed pressure in the system and that's normal. I released the pressure and noticed a couple of days later it built up considerable pressure again. So I released the pressure again. A couple of days later it has built up again? I haven't started the car for over a week and it still does this. I'm thinking there's a chemical reaction taking place. The additive did make a difference in cleaning up the stained water after the initial fire up so maybe that's what's taking place? I'm going to find one of those red leaver, release radiator caps if this keeps up.