Over heating & building up excesive pressure

First put the thermostat back in. It is supposed to restrict coolant flow so the hot coolant has time in the radiator to give up its heat. Right now hot coolant is going right back into the engine. The next thing is to have your mechanic perform a chemical test at the radiator to check for a leaking cylinder head gasket. If that is found, nothing else will solve the overheating. If combustion gases aren't found, you will have ruled out the most common and expensive cause.