Replace it. The tank monitor that Hunter installed is obsolete, plus it never worked very well on a lot of Hunters because the guys who installed 'em didn't follow the installation instructions.
SCAD tank monitors
Scad Tank Monitors are top rated, not expensive and available from a number of retailers including Amazon. It also uses sensors that go on the outside of the tank. You should be able to use the existing wiring, which will make replacement a fairly easy job.
--Peggie