Meanwhile, anyone have any ideas on how to apply Vaseline to a valve
seat located some distance up an 8 mm tube?

Know any pilots who are gynecologists?

How about a syringe thing that is used to give babies an oral dose of liquid
medication? Hot glue a small stir stick or small tube of some type, and it
should be able to get up there to the source.

I also might make a suggestion to use air tool O-ring grease. It is a long
chain grease that acts almost like it has long fibers in it. It should stay
around much longer than Vaseline.
--
Jim in NC