Just been there, just done that......
       Mine just started leaking recently.  I tried snugging it down a
little, but that didn't help.  I took it off, and found the rubber o-ring was
a bit stiff.  It actually made a 'clink' when dropped on the counter.  Of
course, under that rotten o-ring was a crack in the plastic sensor housing.
Boo-hoo.
       My fix:  Filled the inside of the sensor with JB Weld - sealed the
crack.  Bought a new o-ring at the hardware store ($0.45).  Then, just to
make sure, used RTV silicone sealant when I installed it on the threads and
around the o-ring.  It no longer leaks.  I do not plan on removing it again.
If it leaks again I will encase the entire tank in a 12" x 12" x 12" block of
concrete.

       Hope this helps.

Bill Marshall
85 GL Tiico
Phoenix AZ