Hydraulic Brake System . When the brake pedal is depressed, a piston in the master cylinder applies equal force to all wheels, driving brake fluid through steel lines. The pressure of the brake fluid causes the pistons in the wheel cylinders to be forced outward, pressing the pads against the discs or drums.
Air Brake System . When the brake pedal is depressed, a valve delivers a certain amount of air pressure to the brake chambers at each wheel. There are three different air brake systems: service brake, parking brake and emergency brake.