If I undrestand your description, when the resevoir is empty, the float lays on the bottom of the resevoir but, the pump doesn't shut off, after water is pumped out? You have to look at this as opposite of a toilet tank filling with water. In a toilet, the water rises and the foat turn the water flow off. In a sump the water will rise, and the float will turnthe pump on until the resevoir is empty, then it will open the system and turn off the pump. If the float doesn't rise, the resevoir will overflow and the pump won't come on at all. If the float rises and the pump comes on but, the float does "not" drop, the pump will continue to run after the resevoir is empty. http://www.zoellerpumps.com/ProductByCategory.aspx?CategoryID=3
This is Zoellers link. I don't see your pump but they do have a support link. If the float rises and falls with the height of the water, turns pump on, then drops "all the way" and pump doesn't turn off, the switch is bad not the float. Make sure there is nothing in the way of the float dropping to it's full down position.