Coolant Pressure

[Ncostello]

I recently installed a new radiator, hoses, Cummins thermostat etc etc. I filled up the system and have ran it several times up to temp. Everything seems fine. Except even after sitting for a few days....the upper hose is still "solid" and I can't pinch it together when cold. And my overflow bottle level never moves.

I chose a 13psi Stant cap. I decided to try a 7psi cap and see what happened. When I remove the cap cold...I got a purge of coolant as if under a little pressure.

Is it possible I filled it completely full with no air whatsoever? I did this while it was warming up the first time...then I put the new cap on.

Kinda makes me think head gasket failure is causing pressure in the system. But I have no other indications of such a failure. Can pressure get in the system and yet no coolant backflowing into a cylinder?

This is a stock truck and shouldn't have a gasket failure. Boost has always been stock level.

Besides this issue, what cap should I be running? 7 or 13? Factory was 15 and was still on the truck when I did this work. I know the higher pressure gives a higher boil point...but I'm at elevation 520 and probably will never see anything too much different with this truck.