After you shut off your engine, the 7 psi gradually drops over the next few minutes. It only takes around 12 minutes for the system to develop a vacuum relative to atmosphere. At that point the little valve in the radiator cap opens and allows fluid or air to flow into system. (Check out your rad cap, can you find both valves?)

 

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Al, if your cap is a 24 psi cap which you said you could run with full bottle. Why does it develop a vacum on cool down. I have never noticed this on my system, but on the other hand probably never looked at it 12 minutes after shutdown