That test lets you know if the vacuum pump is doing it's job, closer to 20 is good. I think the higher the revs, the higher the vacuum.
You may be able to rig up the mity-vac to the booster itself via the main line, take it off and hook up there, pump away, see if the booster holds vacuum.
Your car is 25 years old, some items will be brittle.

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83 SD

84 CD