1) A brake servo uses negative pressure from an Otto engine's intake pipe or a vacuum pump for diesel engines to operate. 2) The brake servo has two chambers - when the brake pedal is not pressed, negative pressure is in both chambers and they are sealed. 3) When the brake pedal is partially pressed, a spring disconnects the chambers and atmospheric pressure enters one chamber, increasing pressure there based on how hard the brake is pressed. 4) When fully pressed, the chambers are fully separated and atmospheric pressure is in the one chamber for maximum braking force.