a short and simple answer is this, the brake booster uses vacuum produced from your engine to help assist you with your brakes. it helps move the brake pedal in turn pushing the piston in your master cylinder; which pushing brake fluid into the calipers to move the pads towards the rotors making the car stop. flew!Hello IS community, this is another rookie question. What does the brake booster do and i heard it runs out. Can anyone help me on how to replace it or what I need to do to maintain that part, and is it crucial? Thank you IS community.