If the voltage goes away when you connect the wire to the pump and comes back when you disconnect it, Check the battery voltage directly on the battery posts while the wire is connected to be sure the battery has full, normal voltage on its terminals. My next troubleshooting step would be a wire direct from the battery positive post to the pump, to see if it runs. I suspect that it will, but verify.
Your symptom sounds like a high resistance somewhere between the battery and the pump positive terminal. Could be corrosion inside a terminal that's attached to one of the wires, or corroded terminals somewhere in the path, or corroded contacts in a switch or the fuel pump relay, if it has one.
If we assumed that the circuit path was good, then the pump would have to be shorted to cause voltage to go to zero when the supply wire is attached, which would at minimum cause the wire to get quite hot and/or blow the circuit protection device.
