Let me rephrase. Your problem is most likely the pickup foot in your fuel tank. The very bottom piece of the fuel pickup assembly. They're prone to falling off completely, which leaves a gap between the line it attaches to and the bottom of the tank. You'll notice it around 1/4 of a tank.
Try putting some diesel in it, maybe you can get the level past the line and get fuel to the pump again. When you disconnect the pump inlet line, fuel should run out constantly regardless of whether the key is on, or whatever. You aren't getting fuel from the tank.