@bing I suppose we have to define vacuum goes down, my interpretation is if you had -15 psi -12 would represent pressure going down. I honestly could be wrong bu have operated under the concept the higher the rpm the higher the vacuum ie -psi. Remembering vacuum advance for points. Going to your message soon, I don’t know everything either.
Oops, missed the comments on page 2.
Glad you got it solved. If something like this happens again, consider to take your car to the local radiator shop instead of your mechanic. I’m not dissing your mechanic. But those radiator shop guys deal with this kind of problem every day, have all the testing equipment and diagnostic & repair tools, and might get you to a solution quicker and cheaper. Best of luck.