One thing I've found is when you cut your hole, DO NOT disturb the filler. If it looks like you pushed it in, take a needle and push the filler downward towards the
threads...do it very gently. It seems to me that the filler somehow gets moved upward which causes the leaking through the airhole.
Now, when I dremel my slots, I gently push the filler downward and all leaking has stopped for me.