Oil leak at trans yoke vent hole

[Mike Reed]

I have a 92 chevy pickup - owned since new. Suddenly while parked on a level surface it began leaking trans oil out out the front yoke vent hole. I could easily seal up this hole, however I read on another forum that inside these trans is an o-ring that is supposed to seal the yoke.
Would sealing the hole create a problem? or is there an o-ring inside?
Thanks,
Mike