I have tested the cooling system hot and cold, running and not. It holds very steady at up to 15 PSIG. Am I correct in my assumption that when I fill the system I sould end up with as much coolant in the radiator itself and then the expansion tank filled to the level indicated depending on the temperature? What I have been doing is filling this way and When the car is returned to me, after a reasonably short drive, 4 or 5 miles, the radiator level is down to the top of the engine again. Oddly when I take it out for a drive this has not occured? I have exchanged the cap with a used one that I have tested. There is no oil slick that is noticable and the bubbling that exists looks like it is the air that is introduced everday when I have to refill. I should, as you suggest, buy a new cap and see if that has any effect. If this does not end soon I think I might buy some shares in an anti-freeze company! Thanks