My understanding is that the tank is freely connected (no valves or pressure traps) to the radiator overflow and that any fluid that would expand and vent to prevent pressure buildup flows to the expansion tank (that is in turn vented to open air) so fluid moves due to expansion, but no significant pressure builds anywhere in the system and that as the fluid cools/shrinks and would try to create a vacuum (except that it is still vented to atmosphere) the fluid returns to the radiator to avoid creating an actual vacuum.
In other words, there shouldn't be any significant pressure (perhaps a fraction of a PSI) build up anyqhere in the system since it is all freely connected together and open to the surroundings through the vent in the expansion tank. Am I making sense?
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