So crimping the return line where it enters the top of the filter bowl fixed it, at least for a solid hour of running the tractor at high throttle and mowing a sense field. The air is DEFINITELY coming in from that location. And the only logical place to access that air has to be from the return line to/from the tank. I theoretically could run the tractor this way indefinitely, with that small return to filter bowl just closed off permanently…right? All I think I would be sacrificing is the warming effect of the recycled fuel from the injectors to prevent gelling in really cold weather. But I use an antigelling additive in the winter and have a block heater for when it gets really cold here.
I think I will try installing the check valve on that return run to the tank. Providing there is enough fuel pressure to keep the valve open when it heading back to the tank, it should stop the suction of air back the other way. Not a bad experiment for $7.99 either way. If it doesn’t work, I think I’ve still essentially solved the problem!