Thanks guys! I really appreciate the info. I suspected that it might be an air bubble, but then I had people tell me that air should work its way out of the system eventually. I didn’t know who to believe. Since it is under pressure, it seems logical that an air bubble or two stategically placed in the system COULD remain and cause problems. Could you not also purge the system by parking on a steep hill and simply running the engine with the radiator cap off for a few minutes? Will that not accomplish the same things as loosening the top radiator hose?