I had a Festo on the suction side once and it sucked air. Removed the Festo and all was good.

I agree that the tanks should be run in series.

I also no longer use a UAT. I just use a pleated paper filter as a clunk and this has eliminated several potential leak areas from the UAT. I recently flew to long and when I landed the clunk was only submerged half way in the fuel, but the engine never missed a beat.