It could be the lock out switch. When switches and relays get old and lose efficiency, the resistance goes up. This causes problems in the circuitry. I had a tank selector switch (on 90 F-350 front to rear) go bad and it made the rear tank fuel pump stop working and only gave partial current to the front tank.
I thought the fuel pumps went bad, but I pulled the old one out and tested it and it worked. So I went through all of the switches and relays in the circuit and I jumped the tank selector switch and that was it.
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