Adding to Bernard's post, one more thing that I do when I have a manifold that won't seal is to apply a bead of Permatex Ultra-Copper sealant between the manifold and the gasket, applied to the manifold. After tightening the bolts, allow the engine to set for 24 hours before starting, this allows the sealant tu cure. You can also trim off the excess with a knife. So far, this method has cured all but one exhaust or intake leaks, and that manifold got replaced.
