With the engine running, spray carb cleaner strategically around the engine compartment to find the vaccum leak--when you find it, the engine rpm will go up as it draws the carb cleaner into the engine--this will identify where the leak is as it might be a hose in lieu of the gasket. The valley gasket is slightly more involved of a change, but totally doable. the intake plenum is indeed sealed with hylomar. But again, first detect where the leak is coming from and pay careful attention to all the vaccum hoses (such as those to the brake servo and all those running into the front of the plenum). Although not unheard of having a gasket leak, it is usually a hose.