If the filter is horizontal it will trap a pocket of air. Air is compressible. As long as the engine is running, and the pump is pumping fuel, everything works. As soon as you shut off the engine, the pressure will begin to fall and the air-pocket will expand. If the float valve opens, the compressed air will force the fuel up into the bowl. If the float valve remains closed, the pocket will try to force the fuel backwards thru the pump. That's not supposed to happen.
If the pump is sucking air somewhere, It will drive it up to the bowl , until the pump is sucking nothing but air.
IMO, your pump either cannot deliver enough fuel or it is not delivering pure fuel, and in either case, the fuel flow test will determine the next course of action. You, or we , can keep guessing at the solution til next Tuesday, but no one can move ahead with authority until the flow-test results are in. And that's my opinion.
