i have a mazda bongo thats started to overheat after 12 miles , theres no leaks anywhere the radiator pipes are hot on top but cold on the bottom is this the radiator / thermostat or head gasket as there is no exess coming out the exhaust either the oil cap is clean i draind the radiaor and pure rust coloured water came out even after the 3rd flush tried bleeding it too constant air bubbles any help plz cheers Smoggy