You can measure vacuum directly at the vac pump with nothing else in the circuit at all. If the pump tests good (18-20"), add things in and see that vacuum doesn't drop off from a leak. You can connect a MitiVac directly to the brake booster and it should hold. It may take some pumping because the booster is large compared to the MitiVac but it will pump up. Make sure any leakage isn't due to faulty connections or lines going to the test equipment.

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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.