Jamesr, I did the same thing with a 77 old style bronco with two tanks. I was testing it for a week before buying. Drove one tank empty and flipped the switch on the dash to go to the other tank. Little did I know, it had a dash switch for the electric gauge and a mechanical valve coming through the floor below the drivers seat to actually switch tanks. So gauge said full, but not gas flow. Engine died. Killed the battery trying to crank it, got dark, called a tow truck, paid to have it towed, and two days later found the other valve. Doohhhhhhh! I ended up buying it. I finally sold that bronco about ten years later. Sometimes, I wish I still had it, until I recall how it wanted to roll over at highway speed (lot worse than my cj3a), so probably good I do not still have it.
