But why is it that the first port has a fluctuating needle reading while the second port is steady? Is there a valve between the two?
From experience I've worked with several vacuum pumps before (non vehicular - more like laboratory pumps - belt driven, direct drive, etc) and the vacuum gauge hooked up to them usually show steady readings, not fluctuating. Hence my question. The pumps I worked on are rotary with two rotors, two vanes, stators, and a center shaft that turns all these via either belt/pulley combo or direct drive into motor shaft w/ bushing. That's why I was asking if the fluctuating needle is a bad sign. But you're right I read on the pages that 20+ in/Hg is a good measurement.
The ports I'm talking about are the two nipples on what seems to be a valve on the main vacuum hose:
First would be the one to the front (left), the second port would be the one closer to the brake booster itself (right).