The necessary components of the simplest possible air brake system are as follows: (a) a compressor to pump air; (b) a governor to control the compressor; (c) air lines to allow the pressurized air to flow between the air brake system components; (d) a reservoir to store the compressed air; (e) a foot valve that sends compressed air from the reservoir to the brakes when you press the brake pedal; and (f) the foundation brakes, one at each wheel.