My cock was leaking
2000 640 Enduro with the 18 liter tank. I had replaced it (fuel cock) a couple of years ago for the same reason.
I just had the tank off, drained and cleaned (for another issue) and got everything back together and the damn thing would not stop dripping in the "on" and reserve positions.
I took it apart several times and the o-ring looked good. If I put pressure upwards on the dial it would stop.
Finally I tried taking the little spring out and it is now dry as a bone. Doesn't seem to be any problem not having it in there, other than a lack of feeling the notch at the "off" position.
Just FYI if anyone has the same issue.