I've got sensors up the wazoo....some on my plane too. I record this info every few milliseconds. Calibrated sensors. So here is what I see.
If I want my cooling system to operate at 24 psi, all I have to do is fill my swirl pot to the top with coolant. If I don't want it to see that high pressure, I just leave around a cup of air at the top of swirl pot. With my cup of air, it never exceeds 7 psi. Why? Air is compressible, coolant is not.
When you first fire up engine, pressure is 0, relative to atmosphere. It takes around 8 minutes for pressure to slowly climb to 1 psi. The fluid level starts to rise too. All pressure increase is due to expansion of coolant due to heat. Coolant is not compressible, and can greatly increase force against the contained cooling system.
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?)
Logically, from that point on, you can't have a pressurized system. There is no added energy supplied to the system. With one exception: If you have compression leak into cooling system. But even that should have limited duration.
Also, if you had a cooling sys that was normally under pressure, then none of the vehicles would be able to replenish their coolant. They rely on the atmospheric pressure to exceed the coolant pressure 12 minutes after shutdown. That's what forces that liquid in our "overflow" bottle to enter radiator.
I'd double check my gage calibration if seeing pressure on startup.
When I did ground testing with compression leak, this pattern would change. 1 second after full throttle, the pressure would jump to 24 psi, fluid level max out, temperature still cold. This would last for only 5 seconds. This entrained air then had profound effect on the entire cooling sys. I think that was the most interesting part.
Some of the coolant is going to vaporize. This pressurises the system. Some of that vapor will never go back into solution so there should be pressure in the system even when cold.
Dave; this may be the weak link in your logic chain. Why would some of the vapor not re-condense? I hope there is some other reason for the pressure. I'd hate for you to have to tear open the engine.
Al Gietzen