Are the oil return holes clogged? Some valve cover gaskets have holes you have to line up or the gasket covers the oil return holes. If you have pressure coming from the combustion chamber into the valve cover you have other problems.
I suggest pulling the valve cover off and check the gasket, if that was installed right, check and clear the return holes, then start the engine and see if you have pressure coming from some where. Only run the engine for a second, long enough to see what's going on...