Does anyone else's gas cap leak fuel regardless of how much gas is in the tank?
The problem is evaporation and condensation. A fairly common problem with vented gas caps, of which, the DF cap is marginal quality at best. But the cure, or should I say, the treatment is a simple one.

Run out to your local Dollar goods store and pick up a package of girl's ponytail bands. The small elastic bands covered with fabric. Wrap one or two round the cap while it is still locked to the tank. The fabic will wick-up the vented gas fumes so they do not condensate and drip. The band's fabric will air dry on it's own. The elastic bands will tuck-in nicely under the edge of the cap, so they will not be visable until you remove the cap.
/Mike