On my 05b we found that the coolant return bottle nipple was broken. Turns out a forum member had a spare one he gave me. Harley has a new part and bottle for us to buy at 40-50 dollars. Check there first and see if thats the issue its the bottle under the right side cover near airbox. Other than that check all your hose clamps make sure they are super tight. Almost to where u worry u will break them. People say that the thermostat area can be an issue. Also check the radiator fill cap located under the airbox cover thing make sure thats nice and tight.