You might just need a check valve for your pump
Vac Pump Check Valve - Fastlane

You can check whether it's the pump or the booster, but plugging all the small ports on the main vac line using your mity vac gauge on one of them. The run the car. Remove the main vac line from the brake booster, I think it's a 17mm, maybe bigger. Then plug that end of the vac line tightly. See if you vac that still jumps all over or starts to hold steady above 20". If it's still jumpy or reading below 20" your pump needs some attention.

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