You just discovered that sometimes you can actually fix the sending units! The metal wiper gets bent and opens up resulting in no contact to the wire-wound resistor. I have also seen corrosion that can sometimes be cleaned up and things work again.
what seemed to happen was the arm from send unit has 2 bushings on each end that hold the rod there seemed to have play in it from years of goin up and down i noticed that if i held the arm at a certain pos it worked and when i moved it other side it didnt work. so i moved the sweep so it would contact on each side of the played out arm.