This is what they look like (in s13 anyway):
That little float pivots an arm that is rubbing against a series of different points on a circuit board. When it is low, it rubs on the lower points, giving a lower reading. When its high, vice versa.
Check the tracks on the board and make sure its making contact through the whole range. You should be able to test this with it out of the car just plug the fuel pump loom back in and have a play.