Hello Jerry -
I used General Motors gasket sealer (bought it at a Chevy house) and sealed the gasket to the head. I put the valve cover on the gasket with NO gasket sealer (so I could remove it to adjust valves). I had to drive the truck several times and each time, after the engine heated up. I turn the nuts on the cover to pull it down tighter and tighter. Finally, after about five-seven times, it quit leaking. By tightening the cover, I eventually made a deep enough groove in the cover to cause it to seal!
I know it's not very scientific, but it worked for me. Try it!
Joe
