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Typically, if the pedal goes to the floor, either quickly or fades down, or if you have to pump the pedal, it's the master cylinder. If it takes more pressure than usual to make the pedal push, it's the vacuum booster.
Make sure the booster is getting plenty of vacuum. Seeing the diaphragm move isn't an indicator, as it has no load on it. Try pushing against the diaphragm when it's moving; might find a leak or hole in the diaphragm.
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