My Northern Lights genset once did the same thing. I could see airbubbles in the secondary filter pinned to the engine. The air leak was coming from the manual priming pump. No matter how hard you screwed it down, it would still find air. So i screwed it down with large pliers and then coated the entire seal in gasket glue. Since then no trouble at all. Not a single leak. I do not need the manual prime as the fule system has a priming pump on the manifold which i use to prime if ever necessary. Hope this helps.