Pressure coming from valve cover breather during idle?

[Goat Blower]

You're probably thinking of the intake manifold which is at 18-19 in/Hg, although there is a minute amount of vacuum in the oil system at idle as well, but a lot less.

If you've got a lot of positive pressure in your crankcase, you're getting a leak from one of the combustion chambers. Sounds like a bad ring in one of the cylinders, might want to do a compression test to be sure.