Is it possible that there is some amount of restriction in the return lines back to the tanks? In other words, there is a slight restriction that allows the fuel to "build up" a little more than normal in the return lines before it can makes it way back to the tanks? If the fuel was not allowed to flow freely back to the tanks the returns on engine would leak because they don't seal all that well in the first place. Sounds to me like you are **** the returns on the engine properly - this is the only other things I could think of. Kinked return hose farther back on the frame??? Could a malfunctioning FSV slow down return flow back into the tanks?? Just my thoughts. Hang in there - I thought I would never get rid off my air intrusion problem, but eventually I did. I think I installed 3 return kits before I finally got everyhting right. I put on actual Ford return caps too, and they worked, but they had burrs on them too that I had to clean up.