OK CDL Air Brakes key terms

The brake system a driver applies with the foot pedal to slow or stop a moving vehicle during normal driving. In air brake systems, pressing the treadle sends air pressure to the brake chambers.

A mechanical or spring-applied system that holds a parked vehicle stationary. In air brakes it is typically spring-actuated and releases only when air pressure compresses the spring.

A backup stopping system used when the service brakes fail. In air brake vehicles it usually consists of spring brakes that apply automatically when air pressure is lost.

An engine-driven pump that draws in air and compresses it into the vehicle's air storage tanks. It cycles on and off as governed pressure is reached and restored.

A device that monitors air tank pressure and controls when the compressor pumps or idles. It cuts the compressor in at a low set point and out at a higher cut-out pressure.

The upper pressure limit at which the governor stops the compressor from pumping more air. It is typically around 120 to 125 psi in a properly functioning system.

The lower pressure at which the governor restarts the compressor after pressure has fallen. It is typically about 20 to 25 psi below cut-out, often near 100 psi.

A storage vessel that holds compressed air for later use by the brake system. Tanks are drained of moisture and oil through drain valves during pre-trip inspection.

The first reservoir to receive air from the compressor, where most moisture and oil condense. It supplies air to the other tanks but does not directly feed the brake chambers.

A valve at the bottom of an air tank that lets accumulated moisture and oil be blown out. It is opened manually during pre-trip checks and often automatically on some tanks.

The buildup of water, oil, or sludge inside air reservoirs. It can corrode tanks, freeze in cold weather, and damage valves, so tanks must be drained regularly.

A device that adds alcohol vapor to the air system to prevent moisture from freezing in valves and lines. It is used in cold weather but is not a substitute for draining tanks.

A cartridge or canister unit that removes moisture from compressed air before it reaches the storage tanks. It reduces corrosion and freeze-ups and must be serviced per manufacturer intervals.

A pressure-relief valve on the air system that opens to release air if pressure exceeds a safe limit. It protects tanks and components from over-pressurization.

A one-way valve that allows air to flow in only one direction. It keeps air from escaping back out of a tank when the compressor or another circuit loses pressure.

The foot-operated brake valve that controls how much air pressure reaches the brake chambers. Pressing the treadle increases pressure; releasing it lets air exhaust and releases the brakes.

A canister where air pressure pushes a diaphragm and pushrod to apply the brakes. Each chamber converts air pressure into mechanical force at the slack adjuster.

The rod that moves out of a brake chamber when air pressure is applied, transferring force to the slack adjuster. Its travel distance indicates brake adjustment condition.

A lever that connects the brake chamber pushrod to the brake camshaft and takes up clearance as linings wear. Its angle and travel are checked during pre-trip inspection.

The setting of clearance between brake linings and drums so that pushrod travel stays within limits. Poor adjustment lengthens stopping distance and can cause a violation.

A brake chamber with a strong spring that applies the brake when air pressure is released. It serves as the parking brake and applies automatically if air pressure drops too low.

The dash-mounted yellow knob used to apply and release the parking and spring brakes. Pulling it out exhausts air to apply the springs; pushing it in recharges air to release them.

A brake system with two separate air circuits, usually primary and secondary, so a failure in one circuit still leaves braking ability in the other. Each circuit has its own tank and warning.

One of the two separate air circuits in a dual system, typically supplying the rear brakes. It has its own reservoir and can still provide braking if the other circuit fails.

The second air circuit in a dual system, typically supplying the front brakes. It remains functional when the primary circuit loses pressure, allowing a controlled stop.

A visual and audible alert that activates when system pressure falls below a safe level, usually around 60 psi. It warns the driver to stop before spring brakes apply automatically.

A dash instrument showing pressure in the brake circuits. In a dual system it typically has two needles or readouts so the driver can monitor each circuit separately.

A sensor that triggers the warning buzzer and light when air pressure drops below its set point. It may also activate a wig-wag brake light feature on some vehicles.

The time needed for the compressor to raise tank pressure from cut-in to cut-out. Excessive build-up time can indicate a leak, worn compressor, or governor problem.

A check performed with the engine off and brakes applied to see whether pressure drops faster than allowed. It reveals leaks in lines, valves, or chambers before they cause failure.

A pre-trip test where the system is charged, the engine shut off, and pressure watched with and without the brake applied. Allowed loss is typically no more than 2 psi per minute with brakes released.

A test of pushrod travel with the brakes fully applied to confirm the chambers and adjustment are within limits. Excessive stroke means brakes are out of adjustment or a chamber is faulty.

Loss of braking power caused by overheating of drums and linings, often from repeated or prolonged braking on downgrades. It may feel like the pedal goes soft or stopping distance grows.

A technique of applying the brakes firmly to slow several mph, then releasing them to cool while the engine brake holds speed. It is repeated down a grade instead of riding the brakes.

Controlling speed on a long or steep descent by selecting a low gear and using intermittent braking rather than continuous pressure. This avoids overheating and brake fade.

A valve that senses a rapid loss of air pressure in the service line and applies the trailer or vehicle spring brakes automatically. It provides a fail-safe stop if lines separate.

A valve that closes off the tractor's air supply to the trailer if a trailer line breaks or separates. It keeps the tractor's air from escaping so its brakes still work.

The authorization required on a commercial driver license to operate a vehicle equipped with air brakes. It is obtained by passing a knowledge test and, where required, a skills test.

A license restriction, often shown as an L or similar code, that prohibits driving vehicles with air brakes. It is placed when a driver tests in a vehicle without air brakes.

The written examination covering general knowledge and specific endorsements, including air brakes. Passing the air brake portion is required before taking the skills test.