My PTO seals are linking a little also. Maybe with the vent now all that will stop. Well maybe..
By all means check that vent; if it's plugged or clogged, pressure build up will cause leaking at the point of least resistance.
However it is typical for PTO shifter shaft seals to leak. Quite honestly the seals used by the military are far from high quality. Instead of the double lip seals of today, they used seal types that have an inner lip of either neoprene or leather and an outer felt wiper seal. The thing about the outer ring of felt is that the felt will attract and hold water like a sponge. This trapped moisture in turn will cause rust pits on the unplated shifter shaft. Once rust pits are there, replacing seals all day long won't stop the dripping. New double lip modern day seals are easy to install, but if you see pits on the shaft; it is a waste of time replacing seals until they have a smooth, non pitted surface to mate against.
We have built many of the transmissions and PTO units. An overwhelming majority have rust pitted shafts. We install new stainless steel shifter shafts along with new double lip seals. This will solve the dripping issue once and for all since rust pits don't occur on the stainless shafts. This is the only cure for the drips we have found that takes care of the issue for good.