First of all, you can't fill through the vent - that will only result in fuel coming out of the feed line. If you can't fill through that clunk, you'll either have to add a third line, or get rid of the clunk.

Second, the clunk should never go to the front of the tank. In fact, I add a stiff piece of tubing to the clunk line to prevent this from happening in the event of an abrupt stop.