Home › Forums › Stay Dirty Lounge › Service and Repair Questions Answered Here › How to diagnose brake master cylinder vs. brake servo/booster? › Reply To: How to diagnose brake master cylinder vs. brake servo/booster? September 16, 2020 at 7:44 pm #963691 AndreParticipant Hey thanks for the reply. I have a vacuum pump, how much vacuum should I apply to it? Assume I should observe that it doesn’t drop? Do I need to press the brake pedal,