When I repowered from the original Atomic 4 to a diesel I kept the monel tank. In place of the vent line I put large 90* fitting which was drilled and tapped in the side for a hose barb. I had the vent line going out the end of the fitting and the fuel return line plumbed into the side. The engine ran fine but every so often it would slowly die as if it had run out of fuel even though the tank was usually better than half full. I would always change the primary filter even though there was never any evidence of excessive dirt in the fuel. Fortunately the engine is self bleeding and after a few minutes it would restart and run fine. The only thing I could think of was that air was getting into the fuel system somewhere. One fall, after being plagued by this problem for a few years, I decided to deal with once and for all. I removed the fuel tank, the filters, lines, valves, basically everything from the tank to the injector pump. I took the +40 year old tank to a local fabrication shop where we discussed having a new one built. Matt cut it open and pronounced it good as the day it was made. He didn't find any excess crud in the bottom or any obvious pin holes in the pick-up tube. Matt welded it back up, fabricated a new pick-up tube and added a new dedicated fitting for the return line with a drop tube. I reinstalled it that spring along with a new Racor 500 filter, shut off valve hoses, etc. pretty much everything. I never found the smoking gun and I haven't had that problem again but I suspect the was the way I had plumbed the return line. But all that being said, if yours hasn't caused problems I wouldn't worry about it.