Rear Fuel Tank Problem

[DieselChemist]

Guys, the problem with the tank is that I only get about 6 gallons of use from it. I know it holds more. I read about the pickup's getting screwed up and not drawing all the fuel but what I noticed with mine is that the fuel guage reads full when full but when you start using the fuel it drops alot faster than on the other tank. It will go down to empty and thats when I can fit about 6 gal into it. My question is, is this a pickup problem where the extra fuel cannot physically get picked up or is this a sending unit or gauge problem where there is still fuel in the tank that can be picked up but it is just not reading.
Thanks
Bill

 

[6 Nebraska IDIs]

Sounds like the sender is bad.
You'd be surprised how crappy those are.