There is a little tiny nut at the bottom of the aluminum cylinder. You have to first bend down the retaining tabs and then remove the nut. The cylinder just slides off, exposing the insides. Which is a pair of resister wires (NiChrome, I believe) and a float with a set of contacts that "short" out the wires at the float level. As the float lowers along the wires, more of the wire is used in the circuit, increasing the resistance. A full tank (float at top) is about 3 ohms, while an empty tank (float at bottom) is about 90 ohms. Often the problem is a poor contact where wires connect to the terminals. If the wire is broken (and be careful, they are a bit fragile) new wire can be bought (I have read) or sometimes it can be repaired.
If you still need help, ask and I will try to find time this weekend to post pictures of a sender.