The vacuum pump is on the front of the engine, it either fails or it doesn't.

If you are going to throw some more parts at it I would replace the master cylinder and all the soft brake lines. They are getting old, not a bad idea to change them.

If your mechanic knows about these engines he can test the pump, its rather easy and takes a couple of minutes. Along with the vacuum line, they either leak or the don't.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200