Low Fuel Warning Light

[lrg]

The low fuel light on my '85 td never seems to come on. When I start the car the light will go on so I know the bulb is good. Also, when I shorted the wire at the tank the light on the dash lights up. I just never see it come on in normal use, no matter how low the tank gets. I once even drove around with a gallon of diesel in a can in the back to see if the light would go on if I ran the tank really low. After the gauge read into "reserve" and well below empty I couldn't take it anymore and filled it up, but the light never went on. My wife has run it out on hills (we live in S.F.) when she's parked on a steep hill with a very low tank and couldn't restart (I suspect the intake was high and dry) but even then no light. Do I have a bad sending unit? The actual gauge works fine. Any thoughts here would be appreciated.

Thanks