I used the cork end seals on my Air-gap and haven't had any problems. I was planning on eliminating them until I set the intake down and saw the gap was nearly 1/4" so I figured I'd better use the end gaskets. Something else I do is not torque down the intake fully until after the silicone is dry. I install everything just snug. Then after 24 hrs. go back and torque it down. Never had a problem with one leaking except a dang 454 I did for a buddy. It split the end gasket.:angry7:
Oh yeah. I have used Ultra copper silicone for yrs. now and always works great.
One last thing. You probably know this since you said you think your pcv system is ok but you do have the breather on the opposite valve cover from the pcv valve, right? Just checking.