Dead Cylinder Below About 1500 rpm's

Vacuum leak(gasket) or rings/valves? I would think you have to determin if it is mechanical or not, or if it is intermittent. Could the rings be broke again a ring land problem? Did you hear air leaking during leakdown test? Do you have access to a borescope, you could look around at the valves and seats through the plug hole. I would think a bad valve seat would show low compression.