chickenwing-but there is 14.7 acting on BOTH the inside and outside which in effect cancels each other out. As far as the balloon I think that is more of a change due to temperature than altitude. Also most overfill tanks are vented so no "pressure" is being built up in the tank. It's purely the vacuum created in the radiator as the coolant cools and contracts that pulls coolant from the tank back to the radiator.
Goffer, hope I am notstealing your thread. If I am tell me to STHU. 
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'68 Short Step
LS1/T56, Hydratech, Fatman Fabrications Stage III, Baer, Hot Rods to Hell, US Body, S&W, etc
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