Did you run a vapor test on the PTO side? I know a lot of guys around here are experiencing something similar with the pin side of the balancer weights. If you are getting positive pressure at your sample valve, I would say you need to look into your carb. Otherwise, someone just probably installed the ring gear improperly. Sometimes it takes a while for this to cause a proble. Trust me, I have seen it myself!
it should be vented but havent thought to make sure clear and open. tksThe container your valve cover vent line goes to needs to be vented also.