I have fought similar problems on the old l2s that have the snap on filter. We replaced orings and add piece of cardboard between clamp and snap on filter. Check the check valves in hand pump and the check valve on return line coming out top of pump. Quickest and best fix I have found is go to diesel shop and get one of them universal clicking type inline fuel charge pump. Put in line between tank and snap on filter. Your problems will very likely be over