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Not something I've ever done, but I did some searching, and found the following instructions online ...

Testing Procedure for Brake Boosters

Most other sites just talk about having the engine off, pumping the pedal 'til it's hard, then hold it, start the engine and if the pedal drops a bit on startup your booster is fine.

For testing 'off the car' there was one article that suggests using a vacuum hand pump (from a vacuum brake bleeder kit), and pumping until you reach 15-21" of vacuum ...

Home Made Brake Booster Test Tool - See the entry by '79MustangII' near the bottom of the page.
Last edited by 5754
When I broke a part in the booster, I suspected but could not confirm no boost until I actually started the engine and put pressure gauges in the output lines. With engine off (or with broken booster), line pressure was 450-550 psi. With engine running, line pressure was 1100-1200 psi once the booster was repaired.Incidently, this is also an excellent reason to change ANY rubber brake lines (good for about 1500 psi when brand new!) to 3000-psi-test Aeroquip braided-stainless!
How you check the system at home with booster out of the car, I have no clue. Some manuals aay 'listen for air exhausting from the vacuum diaphragm valve'(under the dash). Others say to check vacuum levels. None worked definitively for me. Suggest you take the booster and master cylinder to a specialist shop that works on such things, and let them test it. This will be a real time saver, if not a money saver.
FWIW, the nose section valve that protrudes into the cockpit under the rubber boot was cracked on mine, not allowing vacuum to develop in the big can. No other symptoms.
With the booster out of the car and on the bench, the only thing that you can check is to see if the booster will hold a vacuum. Evacuate the booster to around 20 inches and stop. It should not drop more than 1 inch in one minute. If it drops more than that, then you either have a leaking diaphragm, or a leaking test connection. Also test the check valve.

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