I wouldn't be too quick to guess high pressure, as that shouldn't be an issue. I bet the PCV is clogged and there is probably a little debris around the gasket, or a crack in the gasket causing it to leak there. The PCV gasket also might not be seating correctly in the aftermarket valve cover, if the gasket was meant for a stock steel valve cover, the slight difference in thickness between the two valve covers could cause issues.