You can replace the o-rings on the two shafts on the cover (and the one for reverse) without removing the cover or taking the tranny apart.

When you have the levers off yopu will see a square cross section nylon nylon spacer around the shft. Pick this out with a needle then you will be able to pick out the o-ring. Grease up the new o-rings push them in and put the nylon spacer back in on top and you are done.