Disconnect the main vac line coming from the pump and measure vacuum. Piersburg was an OE supplier and a new pump shouldn't be bad.

You can also try plugging the smaller lines coming off the main line and measure at one of the small lines while leaving the booster connected. You should have ~18' of vacuum. If not, measure directly at the pump. A booster or other leak may look like a bad pump and you want to be sure before spending a couple of Ben Franklins.

One of the competitors was way cheaper than other sites when I was in the market for a pump. Look around.

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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.