When you press the brake pedal, some type of force must be transmitted to the brakes to make them work. Hydraulic brake systems use brake fluid. Because brake fluid cannot be readily compressed, force is transmitted immediately to each brake when you press the brake pedal. Air brake systems use compressed air to produce the force that applies the brakes at each wheel. When you press the brake pedal, compressed air must flow from one part of the system to another. Unlike brake fluid, air is compressible, resulting in a delay of up to 0.4 seconds from the moment you apply or release the brake pedal to the moment that the brakes respond accordingly.