[PatBuckley]

You will notice a tube running from the cap area.....that is to conduct the coolant to the expansion tank when system pressure overcomes the relief on the cap---it then siphons back into the puke tank when the pressure inside the puke tank returns to normal.

You will notice two "flaps" on most radiator caps - the big one is to expel coolant and the little one in the center is so that coolant can get sucked back into the puke tank - which leaves it filled to the top at all times.

It is better to have 100% coolant in a system than 99% coolant....air can get very hot!

On my ERA I mounted the expansion tank on the left side of the engine bay - I got a tall thin one.

Sorry this is so wordy!