I hooked up the Mityvac to the vacuum pump in cold morning ,around 40F, and very low or no vacuum for 2 -3 minutes. During that period, I cannot shut off the engine, brake is hard etc. After the vacuum is built up to 20in hg, I turn the car off and restart, vacuum comes up immediately. It seems to be temperature related within the vacuum pump. It happened last winter but I did not do anything about it. Summer is always OK in the morning.

Any ideas as to what need to be replaced inside the pump? Or do I need a new pump?

Thanks

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300SDL project car

There is a check valve where the main tube meets the pump. When cold, if it is gummed up, it could stick.

I am curious about how you took the measurement and if your brake booster has any signs of corrosion.