Try connecting the upper and lower radiator hoses together with a piece of exhaust tubing to isolate the engine from the radiator, with a tire valve stem in the tubing. Put 10-15 pounds of pressure into the cooling system and see if it drops over time. It's beginning to sound like you're dealing with a leaky head gasket or a cracked head that's letting combustion gas get into the ccooling system. If the pressure drops, pull the spark plugs and spin the engine with the starter. Coolant will probably squirt out of the cylinder with the leak if it's there. You might also be able to listen to each spark plug hole with a length of tubing used like a stethoscope and hear an air leak if the pressure drops quickly.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!