Just Started up my rebuilt 440, everything is looking good except I had to shut it down after 8 minutes due to overheating. The temp got up to 200 before I shut it off, the upper hose and very top of the radiator was hot and full of pressure the rest of the radiator and the lower hose were cold. My first thought of course was a bad thermostat but I was wondering if it could be air bound? Thanks for any help!