from wikipedia An air brake operates similar to the piston in an internal combustion engine would inside its cylinder. Air storage tanks are filled with compressed air by the compressor, which is powered by the engine. When the driver steps on the brake pedal, or treadle valve, this allows air from the storage tanks to flow into the cylinder, pushing the piston down the cylinder. This is the service portion of the system. This cylinder is called the brake chamber. The piston is attached to the push rod, which turns the slack adjuster. The slack adjuster connects the push rod to another rod, which then rotates. This rod is connected to the S-cam. There are several types of air brakes, including S-cam and wedge type brakes. The S-cam are the most widely used type. It is the S-cam that turns and pushes the brake shoes against the lining of the drum brake. The air brakes on a semi-trailer are connected to a tractor with two lines. One line is called the supply line or the emergency line. It is usually larger and is red or has red fittings. The emergency line provides air pressure to fill the semi-trailer's reservoir tank and the pistons that activate the brakes. The other line is called the service line or control line. It is usually smaller and is blue or has blue fittings. In normal braking, pressing the brake pedal pressurizes the service line. This activates a valve in the trailer which directs air from the reservoir and the emergency line to the brake cylinders where it moves the piston that activates the brakes. When the pedal is let up, the service line pressure is decreased. When the service line pressure drops, it causes the valve in the trailer to block the air supply from the reservoir while releasing the pressure in the brake cylinder and the brakes are released. The system is a form of servo or amplifier. If the pressure in the emergency line drops, due to the activation of a valve in the cab, the disconnection of the emergency line coupling, or a break in the emergency line, the spring brakes will come on, because there is no more air pressure in the trailer air tank to hold them back. Air brakes are the preferred type of braking system for these vehicles for several reasons. First, the use of compressed air allows multiple vehicle units to be coupled so that all units have braking capability and so that all of those units' brakes may be controlled from the cab. Coupling would be infeasible if a liquid were used as the mode of transmission of force, as it is in hydraulic brakes. In addition, the use of an air brake system allows for the incorporation of an emergency braking system that utilizes parts of the service brake and parking brake systems. Emergency braking systems are required on all semi-trailers in the United States.[1]