Here is my best guess.  The molecules of any substance (in which they can
freely move) will tend to increase their velocity if they are heated.

increased velocity of individual molecules when integrated over their

number exerts a force on any containment vessel (which we common refer to

"pressure").  As the velocity (heat) of the molecules increases the

is an increase in pressure inside the containment vessel.  If this

increases pass the capacity of the radiator cap then it will force some
liquid into the overflow container.  After the engine stops, the coolant
cools down and the pressure drops (lower molecule velocity) which if it
drops far enough now creates a negative differential pressure inside the
coolant system sucking the coolant in the overflow bottle back into the
coolant system.

Another possible contributor to pressure increase is all liquid has air
entrapped between the water molecules so that t when the liquid is heated
the air molecules come out of solution (much as you see bubbles in boiling
water) and occupy space.  Perhaps the water when heated also tends to

(as its molecules increase their speed with temperature)This generates
additional pressure which causes the water to push past the radiator cap
pressure release.

So there you have it, my 0.02 worth. Perhaps we have someone on the list

really knows what is happening.

Ed Anderson