The brake booster contains an internal diaphragm that separates its interior into two parts, both in a partial vacuum. As the brake pedal is depressed, it causes a valve inside the booster to open, which in turn allows air into one side of the booster. This difference in pressure on one side helps to push the piston inside the brake master cylinder forward in response to the brake pedal, which in turn slows and stops your vehicle. If your brakes are feeling soft then it may be a good idea to replace your brake booster.