Hydraulic Brake System:
Works on the principle that pressure on the brake fluid is applied evenly throughout a closed system. When the brake pedal is pressed, a piston in the master cylinder moves the brake fluid through steel lines applying equal force to all the wheels. The pressure of the brake fluid causes the pistons in the wheel cylinders to be forced outward pressing the linings against the discs or drums.
Air Brake System:
Uses a compressor driven by the engine to fill pressurized tanks with air. When the brake pedal is pressed, a valve allows a certain amount of air pressure to the brake chambers at each wheel. The harder the brake pedal is pressed, the more pressure you give to the brake chambers. There are three different air braking systems: service brake, parking brake, and emergency brake.