Fuel pressure problem..I'm at a loss.

sucking the sock flat? fuel line air leak? can you put a vacuum guage on the pumps fuel inlet and run the motor off another fuel source to see if the check valves are not hanging up? the turret style is serviceable , IIRC there are 3 check valves in those, 2+1 in different directions. I ran across one that had a check valve that had come out (pressed in only) and was floating around in there. Did not produce any useful pressure.