75 f150 dual tanks i was using the auxiliary tank for a long time and i decided to try out the main tank to see if it works. the fuel gauge switch is on the heater control panel and the tank switch doesn't work. i put 40 dollars in gas today today and 2 gallons yesterday i drove probably 35 miles total. as i went to put in 10 dollars more it started coming back up out of the filler. this is a 19 gallon tank that was empty as far as i know a few days ago.
the fuel guage seems to be sticking at 1/4 tank. i was at the gas station feeling like a real idiot with gas pouring out the side of the truck. what i am wondering is should i get up under there and bank on it with a mallet or just continue to use the other tank.

 

I don't have any diagrams of something that old but I am pretty sure that if the dash tank select switch doesn't work, then the gauge won't work either.
The tank senders could be bad as well so, your tank probably already had some fuel and/or water in it.

Kinda wondering if you knew the switch does not work then why put fuel in the other tank?