On my DRZ400 the coolant overflow will fill up & not return to the radiator. In the past it has blown the head gasket causing this as it blows by the water passage. Last ride I lost coolant & over heated the hell out of the motor. What bothers me is the radiator has pressure in it after it has cooled down & it doesn't seem as if it should keep pressure if the head gasket is leaking. any ideas? I'm installing a newer motor in it but want to make sure that I will keep the coolant where it belongs.