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