[steve25]

A system good system will never have pressure in it once it has cooled all the way down, what you heard after thaecar sat for a while was air entering the rad when you opened the cap due to the vacume.

[GStage1]

The reason for the head gasket statement....I have found several engines with the same problem.....removed about 1" of fluid below the radiator filler hole, ran the engine without the cap on and watched as bubbles percolated in the fluid....the engine was pumping air/fuel mixture into the coolant......engines never consumed fluid.....due to the way the head gasket was letting the air enter the coolant. Replaced both head gaskets and no more air bubbles or pressure.

On a normal engine, you should leave a little room for thermal fluid expansion so you don't spill fluid. Once the engine/coolant returns to normal outdoor temperature...there should not be any pressure since the heat is gone.

[STVSHOBRD]

I would think that if thats the case when you shut the motor off and there is pressure in the system, coolant would leak through the bad area in the gasket and after time there would be no pressure left. Sometimes my car will sit for a month without running. I would think if there is a way for air to enter the coolant, there would be a way for coolant to go the other way.