I've had low vacuum since I got my car seven years ago (10-12"). I checked the intake manifold for leaks using all manor of sprays and didn't find any leaks. I had low vacuum (14-15") in a stroker FE engine which was due to a long duration cam so I planned a cam change. As the 351C engine was so extremely low I was sure the cam timing was off or the chain was shot. Today I pulled apart the front of the engine only to find quality double roller chain set straight up. On with the cam swap. When I loosened the first bolt I it made a hollow sound through the intake as did all of them. I've removed many intakes and none made that sound. Seems the only thing holding it down was the silicone on the end. The gaskets were black around the four inner ports. Looks to me like they were leaking from the lifter valley. I bet someone reused the gaskets and didn't tightened in sequence. I'm thinking to reseal and close up the engine and give it a try. Opinions ?