Water dump valve rotary seal

My Nimbus 2C has a rotating shaft that passes through the root rib into the
integral wing tank. This shaft operates the dump valve. This shaft has
about 1mm radial play and appears to be leaking water at a slow rate. I
detect no axial play. I find that the closing spring can be gently turned
off the shaft without damage but this doesn't expose anything resembling a
seal. The shaft appears to turn in the carbon fiber composite although
you'd think they would have used at least a bronze bushing.

One other Nimbus 2 pilot suggested that there may be a rubber boot on the
inside of the wing that is supposed to seal the shaft. Maybe, but I would
suspect an O-ring or gland seal. Is there a quick fix for the leak? I
really don't want to cut into the wing during the soaring season. The
maintenance manual is completely silent on the subject.

Trying a test case on the workbench proves that I can force silicon grease
into the space between a shaft and journal with low air pressure.
Alternatively, if there is a natural rubber O-ring it might be made to swell
by soaking it in denatured alcohol.

Any useful insights folks?

Bill Daniels