I had to post this cure to my problem in the hopes it may help someone down the road. I had installed Autometer gauges in the dash to replace my 69 Ranger factory ones and the fuel gauge was the only one that I couldn't get to work correctly. The gauge has (3) posts - ign.,sender,ground. With key on I have 12 volts coming in and 5 volts going out to sender so I then went to sending unit and removed it from tank. With key on movement of the arm makes gauge needle move so I felt my sending unit was OK. I took yard stick and stuck in tank to get a fuel reading and also seeing how deep the bottom of the tank is from the s/unit mounting hole(14 1/2"). I removed yard stick and lined up s/unit with the accured measurement and moved the float arm based on what fuel was in the tank from the gas on the yard stick to get idea of where the float arm would be in relation to the s/unit. I came up with 1/2 full. I had installed a new brass float with the orginial rubber retainer and when I stuck the s/unit back in tank with key on the gauge read empty. My only other think I could check was maybe the weight of the new float plus the rubber retainer was to much to move the arm up in fuel so I removed the rubber retainer and placed s/unit back in tank and believe it or not; when I turned key on my gauge read 1/2. I am happy with my results but I'm unclear why this rubber retainer would keep the float arm from moving (it looks brand new) or maybe the contact arm inside the sending unit housing can cause this but at this point my gauge is finally working.

 

Maybe somehow you lost the ground to the tank, and then got it back accidentally on the last attempt.