[68LSS1]

Originally Posted by Stocker

Yup - so depending on how you visualize it, either the vacuum within the system sucks the coolant out of the overflow tank, or - when atmospheric pressure exceeds the pressure within the system, it (atmospheric pressure) pushes the coolant back into the radiator. Same difference.

Not the same difference. Atmospheric pressure has nothing to do with it. As a liquid is heated it expands which creates pressure. We release the pressure via the cap to the tank. There is a seperate part of the cap with a return or check valve that is held closed by this pressure. As the system cools it shrinks which creates a vacuum which draws fluid from the tank. I am not following you logic about what atmospheric pressure has to do with it. When you put a tire pressure gauge on a empty tire what does it read? Zero, no pressure. With your line of reasoning I would have 14.7 psi in the tire. Explain to me how atmospheric pressure has something to do with this, I could be wrong, have been before (once in this thread already ) and will be again.