The only other way pressure could get too high in the crankcase or valve covers is to have a problem on a piston or maybe a blown head gasket. With the piston, it could be broken rings, cracks, or hole burned in the piston.
Is it missing or down on power?
A compression test should show if there's a problem.
Pressurizing the problem cylinder and listening for air escaping would tell you where to look for a problem.
If it's running ok, there may be no problem. Could the valve cover have been over-tightened?