I have no experience in this issue, but I think a distributor needs to breathe. Plugging the holes may hinder that. Possible making a felt washer to cover the holes, but not to tight, to allow it not to tear. As some have said, possible to much "Blow-By". Maybe wrong PVC, not allowing enough draw back or vent cap dirty or plugged. Stock ones have a nylon spacer that may help to keep oil from creeping up on the shalf. Something may be missing here.