We've had Fords with:
1. Carburetor and In-Tank Electric Pump. These low pressure pumps WOULD allow fuel to be pulled through them when they were not running.
2. EFI with In-Tank AND Frame-Mounted Pumps. This is what I think you have. I'm pretty sure the Frame-Mounted Pump would NOT allow fuel to be pulled through but I am not sure if the In-Tank would or would not.
3. MPI with only High-Pressure In-Tank Pump. This pump WILL NOT let fuel through when not running.
Vehicle (1.) above was a 1983 E350 with two tanks and carbureted 460. I wish I could find it, but I remember that...
...there was a RESISTOR in the running power feed to the Fuel Pump Motor. In the 1983 chassis, the pump ran at reduced voltage when the key was in RUN due to the Resistor. In START, the Resistor was bypassed. This was common with Points/Condenser Ignition back in The Day, but the trick was also used with these fuel pumps. So...
...very possible there was a change-over and the pump supplied in error was what I'll call "Low Voltage" and meant to be used with the Resistor, when the chassis in question had had the Resistor deleted and needed to run on "High Voltage."
