Zeroing BII Vacuum Gauges

I was getting my Shurflo pump cleaned and ready to put away when I found some plumbing improvements I could make to the manifold at the strainer. I thought I would check this out and get it as good as I could before putting it away. I noticed my vacuum gauge was not at zero but was around 5 in Hg while just sitting there. After a YouTube search, I could not find a way to zero this style of gauge. I took the cap with the lens off and removed the pointer and reinserted it so it read zero. Although I am sure I made improvements, the BII gauge didn't think so. What was a 25 reading on the gauge was now an 18. Ok, that could be... if what I worked on wasn't really hurt my vacuum. After tinkering some more I noticed before running the system again, the gauge now reads 20 - at rest with the system off and valves open. I did the same gauge adjustment as before and now when running, I am getting 10 in Hg for the reading. I am wondering of my gauge zeroing is what is making my numbers drop. I have other gauges I can try tomorrow but still the question - can these be adjusted or are they just bad?

These are not liquid filled. There are two screws on the backside, but they only attach the face to the back.

Sounds like a broken gauge. Put a new one in and buy some extras while you are at it. They break, so it's always good to have a replacement or three on hand.

They are not adjustable. Once the get some liquid in them I find they are screwed. Even if they were adjustable I would trust them. I usually buy a box of 10 every couple years. They are pretty cheap on Amazon.

Sounds like good advice. I'm getting some new ones.