[undy]

My expansion tank stays full, to the top... always. I've got a sealed recovery tank system. As the coolant expands with engine heat it flows from the expansion tank to the recovery tank. When the engine starts to cool it pulls the coolant back into the expansion tank. Pat's system pulls air back into the tank as the engine cools. I just don't like to introduce air unnecessarily into the sealed system. Oxygen tends to enhance oxidation, something you definately don't want in your cooling system.