You can buy a hand held vacuum pump with a gauge for next to nothing. One hour at a shop is $100 and most techs working have no idea how to work on these cars.

Simply plug all of the small lines that come off the main line using a plastic golf tee. Connect your vacuum gauge to one of the smaller branches. Disconnect the end of the big line at the firewall and plug it with something. Your thumb will work.

Start the car and note measured vacuum. It should be ~15 inches or more. If less, make sure you don't have a vacuum leak in the line that is still connected.

Some hand held vacuum pumps come with rubber connectors suitable for connecting directly to the car vacuum pump where the metal line disconnects.

You either have a vacuum leak, bad vacuum pump or bad vacuum cut off. It is a simple system with well documented test procedures. Perhaps go to school - youtube university is one place that may be helpful.

Vacuum pumps were upwards of $300, becoming scarce even new without repair parts available the last time I checked a couple of years ago. You don't won't like paying an uneducated tech to buy, upcharge and install a pump at those prices - say $600 or $700.

These cars are old and thus very expensive for those who can't wrench.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.