I actually bought the gasket for the Pre-Heater, but that gasket seals the coolant at the mating surface between the PH and the head. That is from what I'm told, so that gasket will not solve any problem in reguard to air bubbles. I will be changing the black rubber hose between the hard metal line from the tank and the PH. Since the bubbles can be seen in that first clear plastic line (new) from the PH to the pre filter, air simply has to be entering before that point?