[tsubaki]

The senders are nothing but a rheostat.
Some can wear like sandpaper and read intermittently, completely lose contact and not read at all or stick at any given location and read a constant value.

I don't remember what the resistance values were on those things.
Like spare said: "Take the sender out and hook an ohm meter to it, slowly run the float up and down, it should change the ohm reading in a smooth flow, any jumps or zero readings would indicate a bad sender", as would one reading a constant value while moving the float up and down.