wondering if this might be a small hg leak? when i **** the truck down i can see little air bubbles slowly bubble up in the overflow from the radiator. TBH the overflow has been low forever and i never got around to filling it, could it just be sucking air when its cooling off, then slowly burps the air when its warm? i hasve yet to check it when actually running, and i should probably fill; that oerflow to confirm and do a leakdown check. Just had some free time and figured id ask!