Alloy Header Tank Round This tank is designed for vehicles which do not have a pressure cap on the radiator or require a pressure cap on the expansion tank. (If you do have a pressure cap on the radiator and want to fit this breather tank then you would need to replace it with a valveless cap (see radiator caps) or remove the valve from your current cap) Unlike an expansion tank (which can be mounted anywhere) a breather tank must be mounted at the highest point of the cooling system to operate efficiently. Prevents air locks and leaves room for expansion.