The fuel gauge on my '84 wasserboxer (for a number of years) would only go to about 3/4 when I filled the tank.  In the last couple
of years it would only go to slightly above 1/2.  It finally quit giving any reading at all, and because the temp gauge worked, I
figured it must be a "Sender" problem.

Well .... I removed the gauge cluster from the dash and loosened the nuts that hold the fuel gauge's terminals to the "Ribbon"
circuit, sprayed in some WD-40 and carefully re-tightened the nuts.  The gauge not only works again ... it works correctly!

A little corrosion (?) goes a long way (but no corrosion was visible).

Mike
'84 GL.