['63GENIII]

Hey Cris, might take a flashlight and see if you can see down to the bottom of the tank from the filler hole. Either that or pull the sending unit / pickup (when the tank is low). It should be silver and clean at the bottom.

Many times over the years these trucks are left with caps off or are left with a rag stuffed in them which lets rain water in. Water ends up rusting the tank. Rust plugs the brass sock on the end of the pickup. Worth a look. When I got mine, i could drive for miles on anything over a 1/2 tank. Any lower and there wasn't enough sloshing to keep the rust sediment off of the sock.

Also check the compression connection from the pickup line that's outside of the tank to the one that runs to the pump. You could be losing the prime there even if its not leaking fuel, the low tank level could siphon the fuel back into the tank from that point.

One last thing to check. IIRC, the sending unit is spot welded to the pickup at maybe 1/3 - 1/2 way up the tube. If one of those spot welds cracked, there may be a pinhole in the pickup. Like using a broken straw to drink a soda.

Good luck and let us know what you find.