Not sure if your vent system has a rollover check valve at the end of one of the lines. If you do have one, I'd check to see if the ball bearing moves back and forth freely in it. I would also disconnect the lines at the tank and blow compressed air through them. I poured a shot of gas in mine and blew it out the other end with a rag tied to it. The gas acted like a slug and cleaned it out pretty well.