There are two ways I check for pressure getting into the cooling system the first is one of them exhaust gas detectors , it is a fairly cheap tool that works well , you put a tube into the radiator throat and fill it with some special liquid , run the engine and if it changes colors then you have exhaust gases getting in , either a blown head gasket , cracked or warped head or a crack in the block would be some causes for that , the other tool is a radiator pressure tester , with it mounted to the radiator throat and pumped to the psi of the cap and the motor running it should not build up pressure ----------going back to a clean radiator , are you sure you got all the dirt out with compressed air , it may take a pressure washing , an what about an internally clogged up core -=---- agood test for the themosat is to remove it and open it with a screw driver , put a piece of thread or dental floss thru the opening and tie it to something that will lay over the top of a pot , boil water in the pot with a themometer and when it falls off the string note the water temp , I have not found many times that a thermostat was a probally