Yanmar Fuel Leak
I had the same problem with my yanmar. I took a good look at the threads in the filter case an found them questionable. The 8 mm. screw has been altered to bleed off any air so I wanted to use it. I had already had to rethread another 8mm. thread so I drilled out the threads and tapped the new hole, screwed in some threads, put the screw and washer back in and have had no trouble since then.
You can buy one of these helicoil repair kits at any autoparts store and probably at west marine.
Good luck, Bill