I just connected an ohmmeter to it. One lead to the terminal on the top, and the other to the body of the unit. I could then see the resistance change as I moved the float up and down. I suspect the rheostat is kind of worn. The readout isn't always very smooth and it jumps around a little. Still, at least it's working now.

1938 Chevy 1/2-ton
More info in the Gallery Forum
She looks good at 50 yards, and 35 miles per hour, on a dark night.