Before I sold him the rig, I checked the coolant which was a little low and I added a little. I figured it was from the leak I had at the heater pipe connection. So the radiator was full when he left. Is it a possible senario that the thermostat became stuck closed and caused the engine to overheat and blow the head gasket? Or, can a head gasket slowly degrade and leak for months and then suddenly blow??

Thanks again.