I have been chasing down air leaks in the fuel transfer system. Suspecting the fittings at the tanks, I re-tightened and re-sealed barb and pipe fittings and double clamped hoses at the tanks all to no avail. I even found a loose hose clamp that I swore I had tightened, but that was not the cause.
The mislabeled fuel valves
So, then, why is the pump sucking air? Open the tank inspection port... its empty! But the fuel gauge says full! Re-check hose and tank sender routing to discover: mislabeled ball valve and a mislabeled tank sender lead. Turns out I was pulling fuel from a wrong tank that the fuel gauge said was full that was in fact empty (hence sucking air). Dohb!
The check valve installed at the transfer pump output.
Everything all fine now. Turns out the addition of a one-way spring loaded check valve on the transfer pump output helps prevent air from being sucked back through the pump as the suction vacuum is created. This makes for smoother pump operation, but fuel transfer still takes a bit of time. I am chalking that up to the low throughput capacity of the pump.