My 1983 water-cooled is getting a lot of pressure in the coolant reservoir(head gasket or cylinder
spacer-seal or green o-ring, or cracked head?).  In the process of trying to pin down which cylinder
or side the problem was coming from I hooked up a vacuum gauge and discovered a pretty low reading
at idle (8-10 inches hg).  I'm not that familiar with the use of a vacuum gauge and would like to
hear some ideas about this low reading.  Everything I've read ties a low reading to things like an
intake manifold leak, late valve timing, brake booster leak.  A head gasket leak causes the vacuum
reading to drop sharply each time the leak occurs...that is not happening, the low reading is pretty
steady.

Here is some other information:

Compression test: 140-150 all cylinders
Timing: set at 0 , if I set it at 5 ATDC the vacuum reading drops even lower to 5 inches
Engine rebuilt 10,000 miles ago
Disconnecting individual plug wires, and also injector wires one cylinder at a time causes the same
change in rpm and vacuum reading for each cylinder...it is not evident that any one cylinder is
carrying more of a load than  the others.
Very slight water usage, no steam or vapor coming out at the tail pipe.
Isolating brake booster vac line doesn't change the vacuum reading (ie, no leak in booster).
Spraying Sure-start (ether spray) at all possible vac leak locations doesn't show any change in rpm.

This pressure problem has been occuring for about 6 months and doesn't seem to be getting much
worse, but it is very obvious and there's enough pressure in the coolant reservoir to keep the level
below the coolant level sensor, and to keep the overflow bottle full and bulging a bit.

Does anyone have an idea about this low vacuum gauge reading, and can you suggest a way to connect
it to this pressure problem.  I'd like to know more before I start pulling off the cylinder heads.

John