10 – 15 in of vacuum should be enough to check it for any leaks and no, no need to press the brake pedal.
Keep in mind if the was a leak in the brake booster, most likely you would have a harder brake pedal due to lack of vacuum.
One other item to check would be the check valve on the boosters vacuum hose.
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This reply was modified 2 years, 8 months ago by
Richard Kirshy.