any possibility that you have a combustion leak into the coolant system? EDIT that takes alot of psi to expand em out like that, You could get a "tank valve" from Oreillys for less than $3 & screw it into the cooling temp sensor port & pump it up to say 10 psi & see if it hold pressure (check with your tire gauge). This ain't the same as major compression psi coming from the other direction into the cooling system but it would be a start (& cheap/easy). with the eng full of coolant/not much dead air space it will reach 10 in a heartbeat so put the air chuck from your shop air on there for a split second at a time (it ain't like airing up a tire that takes minutes). The last thing you wanna do is blow up your hoses!