MT CDL Air Brakes key terms

The foot-operated brake system used during normal driving to slow or stop the vehicle. In an air brake vehicle, applying the treadle sends air pressure to the brake chambers.

A brake that holds a parked vehicle stationary. In air brake vehicles it is usually spring-applied and released by air pressure, controlled by a dash valve rather than by the foot pedal.

The foot pedal or valve the driver pushes to apply the service brakes. How far it is pushed controls how much air pressure reaches the brake chambers.

A sealed can containing a diaphragm that converts air pressure into a mechanical push rod force. The push rod moves the slack adjuster, which rotates the brake cam or applies the disc brake.

The lever arm that connects the brake chamber push rod to the brake cam shaft. It converts push rod travel into rotation and adjusts automatically or manually to keep brake lining clearance correct.

The distance the brake chamber push rod extends when the brakes are applied. Too much travel means the brakes are out of adjustment and may not apply fully, so it is measured during inspection.

Setting the clearance between brake linings and drums or rotors so the push rod stroke stays within the manufacturer's limit. Automatic slack adjusters normally do this, but the result must still be checked during inspection.

An engine-driven pump that builds air pressure for the brake system. It is governed to load and unload within a set pressure range so the system stays charged without overpressurizing.

A valve on or near the compressor that controls when the compressor loads and unloads air. It sets the cut-in and cut-out pressures, typically about 100 psi cut-in and 120 to 125 psi cut-out.

The maximum system pressure at which the governor unloads the compressor so it stops building air. It is usually around 120 to 125 psi and should be checked during a pre-trip inspection.

The lower system pressure at which the governor loads the compressor again to rebuild air. It is normally about 100 psi, giving the system a working pressure band between cut-in and cut-out.

A device that removes moisture and oil vapor from compressed air before it enters the storage tanks. Keeping air dry prevents corrosion, freezing, and sludge that can clog valves.

A storage reservoir that holds compressed air for the brake system. Tanks are divided into wet and dry sections in dual systems, and each tank has a drain to remove moisture.

The first storage tank that receives air directly from the compressor and air dryer. Because it collects the most moisture, it must be drained often, usually daily.

A storage tank downstream of the wet tank that supplies air to the brake circuits. It holds the cleaner, drier air used for brake application and for the warning and emergency systems.

A valve at the bottom of an air tank used to release accumulated moisture and oil. Draining tanks daily, and checking for excessive water or oil, is part of routine maintenance.

A brake system with two separate air circuits, usually front and rear, fed from separate tanks. If one circuit loses pressure, the other still provides braking, and a warning alerts the driver.

One of the two independent air circuits in a dual system, typically supplying the rear brakes. A loss of pressure in the primary leaves the secondary circuit available for stopping.

The second independent air circuit in a dual system, typically supplying the front brakes. It allows braking to continue if the primary circuit fails, though stopping distance will increase.

A buzzer and light that activate when system pressure drops below a set level, usually about 60 psi. It warns the driver to stop and find the leak before braking is lost.

A pre-trip test that confirms the low pressure warning buzzer and light come on before pressure falls too low. The driver builds pressure, then bleeds it down and watches for the warning.

A brake chamber with a strong spring that applies the brake when air pressure is released. Air pressure holds the spring off during normal driving, so a loss of air causes the brakes to apply automatically.

The dash valve, often a yellow diamond-shaped knob, that controls the spring brakes. Pulling it exhausts air to apply the spring brakes; pushing it restores air to release them.

A dash-mounted control that is pushed in to release and pulled out to apply a system, such as the parking brakes or a trailer air supply. Its position shows the driver the current state.

A valve that prevents service brake air and spring brake force from combining on the same brake. It protects the brake mechanism from excessive force when the parking brake is applied during a service brake application.

Using the spring brakes or another backup method to stop when service brakes fail or pressure is lost. The driver should keep the vehicle under control and bring it to a safe stop.

A loss of braking ability caused by air pressure loss, a broken component, or overheated brakes. The driver should downshift, use engine braking, and apply the emergency or parking brake gradually.

A temporary loss of braking power caused by overheating of drums, rotors, or linings from repeated or prolonged use. The driver should slow down, use a lower gear, and let the brakes cool.

Using a lower gear so the engine's compression slows the vehicle, reducing reliance on the service brakes. It helps prevent brake fade on long downgrades and is part of safe braking technique.

The total distance a vehicle travels from the moment the driver perceives a hazard until it stops. It includes perception, reaction, and braking distance, and increases with speed, weight, and poor brake condition.

A pre-trip check for air leaks with the brakes applied and the engine off. The driver watches the gauges to see whether pressure drops faster than the allowed rate.

A leak test performed with the service brake pedal held down and the engine off, watching for a pressure drop beyond the allowed limit. It checks the lines and chambers that hold pressure during braking.

A pre-trip check that the compressor can raise system pressure from a low reading to cut-out within a reasonable time. Slow buildup points to a weak compressor, a leak, or a restricted line.

A gauge that shows how much air pressure the driver is applying to the brakes. It helps the driver judge braking force and notice changes in how the system responds.

A defect serious enough that the vehicle must not be driven until repaired. Examples include excessive push rod travel, a failed low pressure warning, or an air leak beyond the allowed rate.

A device that adds alcohol vapor to the air system to lower the freezing point of moisture. It helps prevent ice from blocking valves and lines in cold weather.

The dash valve, often a red knob, that controls air to the trailer. Pulling it closes the trailer supply and applies the trailer spring brakes, which is used when uncoupling or in an emergency.

A valve that seals the tractor's air system if the trailer breaks away or a trailer line fails. It keeps the tractor's air from escaping so the tractor brakes still work.