> I still don't understand what mechanism causes the cap to allow fluid back
> in, though. I can see that the added pressure of the heated coolant will
> cause the stopper to rise letting the coolant into the overflow tank, but it
> seems that as the coolant cools, it would tend to keep the stopper closed
> not allowing the fluid to return. Anyone know what magic makes it work?
Generally there is a metal disc in the center of the rubber stopper that pulls
downward under vacuum to allow fluid back into the radiator.
Randall
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