Engine Vacuum

[MMoore8425@domain.elided]

I have a 71 Stag Mk 1 with a Holley conversion. I went to the Holley
conversion because I had a hard time with the Strombergs. I've had a difficult
time though getting a good idle; which may also have been true with the
Strombergs. My latest thought was that a vacuum leak was the culprit, and
indeed it was. Corrected, it made a tremendous difference in the idle. 
The leak is because of the vacuum line connected to the cam cover. With it
connected, it is as though it were open to atmosphere. Plugging the vacuum
line solves the problem. Is the engine supposed to be air tight? Where does
the air leak into the engine? Through the rings? Seals? What's wrong with this
picture? Any advice will be appreciated.

Mike Moore
mmoore8425@domain.elided