OR CDL Air Brakes key terms

The foot-operated brake that slows and stops the vehicle during normal driving. In an air brake system, pressing the treadle sends air pressure to the brake chambers at the wheels.

A brake that holds a parked vehicle in place and is not used for normal stopping. On air-braked vehicles it is usually applied by releasing air from the spring brakes.

An engine-driven pump that builds and maintains air pressure in the brake system. It draws in outside air, compresses it, and pushes it to the reservoirs through the supply circuit.

A valve that controls the compressor by connecting and disconnecting it from the reservoir. It starts the compressor at a low cut-in pressure and stops it at a higher cut-out pressure.

The reservoir pressure at which the governor stops the compressor from pumping, typically around 120 to 125 psi. Pressure above this point is released by the safety valve if it keeps rising.

The lower reservoir pressure at which the governor restarts the compressor, commonly about 100 psi. A gauge that never drops to this point may indicate a stuck or misadjusted governor.

A device between the compressor and the reservoirs that removes moisture and oil from compressed air. It helps prevent corrosion and freeze-ups in valves and lines.

A tank that stores compressed air for the brake system. Trucks have at least two, and each circuit of a dual system has its own supply so a leak in one does not empty the other.

The first reservoir that compressed air enters after leaving the compressor and dryer. Moisture and oil tend to collect here, so it must be drained often, usually daily.

A reservoir that receives air after it has passed through the wet tank and is protected by a one-way check valve. It supplies air to the brake circuits.

A manually or automatically operated valve at the bottom of a reservoir that lets collected water and oil out. Draining tanks daily prevents moisture from damaging valves and causing brake failure.

A pressure-relief valve on the air system that opens if reservoir pressure rises too high, usually near 150 psi. It protects tanks and lines from over-pressurization if the governor fails.

A one-way valve that allows air to flow in only one direction. In a dual air system, check valves keep a leak in one circuit from draining air from the other circuit's reservoir.

The foot pedal that the driver pushes to apply the service brakes. Its movement is converted into an air pressure signal by the foot valve, which sends air to the brake chambers.

The valve operated by the brake treadle that controls how much air pressure reaches the brake chambers. It is also called the treadle valve or application valve.

A valve mounted near the axles that quickly supplies a large volume of air to the brake chambers when signaled by the foot valve. It shortens brake response time on long vehicles.

A cylinder containing a diaphragm that converts air pressure into mechanical force to apply the brakes. Air pushes the diaphragm and pushrod, which moves the slack adjuster.

The lever that links the brake chamber pushrod to the brake camshaft and takes up clearance as linings wear. It converts the pushrod's straight push into rotating force at the brakes.

The rod that extends from the brake chamber when air pressure is applied and transfers force to the slack adjuster. Its travel is measured during inspection to check brake adjustment.

The mechanical parts at the wheels that actually create friction and stop the vehicle, including brake shoes, linings, drums, and the camshaft. Air pressure only supplies the force to operate them.

The rotating metal drum attached to the wheel that the brake shoes press against to create friction. Worn or cracked drums reduce stopping power and can cause uneven braking.

The friction material bonded to the brake shoes that contacts the drum. Linings wear thin, can become glazed or contaminated with oil, and must be replaced before they damage the drum.

A brake system with two separate air circuits, usually a primary and a secondary, each with its own reservoir and valving. If one circuit fails, the other still provides braking.

One of the two independent circuits in a dual air system, typically supplying the rear brakes. It is filled from its own reservoir so a failure elsewhere does not disable it.

The second independent circuit in a dual air system, typically supplying the front brakes. A loss of pressure in the primary circuit still leaves the secondary circuit able to stop the vehicle.

A brake chamber that uses a strong spring to apply the brakes when air pressure is released. Air pressure holds the spring compressed to keep the brakes off while driving.

A brake used when the service brakes fail, often applied by releasing air from the spring brakes. The parking brake control valve can usually be used for this purpose.

The dash valve, usually a yellow diamond-shaped knob, that releases air from the spring brakes to apply them and admits air to release them. Pulling it out applies the parking brake.

A valve that prevents the service brakes and spring brakes from being applied at the same time with full force. Applying both together can damage brake chambers and linkage.

A warning device that activates when reservoir pressure drops below a set level, usually about 60 psi. It gives a visible and audible alert, such as a light and buzzer, before braking is lost.

A dash gauge that shows the air pressure in the brake system, often with separate needles for each circuit. The driver watches it to confirm the compressor is keeping up and pressure is not falling.

A test that confirms the compressor can raise reservoir pressure from a low reading to the governor cut-out within a reasonable time. Slow buildup points to a compressor, governor, or leak problem.

A check for air loss with the engine off and the system charged. The driver notes gauge readings, applies and holds the service brake, and watches for pressure drop beyond the allowed limit.

A leakage check made with the brakes released and the system fully charged. The driver watches both gauges for a specified time and confirms pressure loss stays within the allowable limit.

A leakage check made with the service brake fully applied and held. The driver watches the gauges for a set period to confirm the system holds pressure under braking force.

A check that the low pressure warning activates before pressure falls too far, usually by pumping the brake with the engine off. The warning should come on by about 60 psi.

A check that the spring brakes apply automatically when air pressure drops to a low level, usually around 20 to 45 psi. The vehicle should not be able to move once the brakes set.

The distance the brake chamber pushrod moves when the brakes are applied. Too much travel means the brakes are out of adjustment and stopping distance increases.

The setting of slack adjusters so pushrod travel stays within limits. Out-of-adjustment brakes are a common cause of failed inspections and longer stopping distances.

The pre-trip walk-around check of air brake components, including hoses, chambers, slack adjusters, drums, and linings. It confirms the system is safe before the vehicle is driven.

The short delay between pressing the brake pedal and the brakes actually applying, caused by air traveling through the lines. It is why air-braked vehicles need more following distance.

The total distance a vehicle travels from the moment the driver decides to stop until it is fully stopped. It includes perception, reaction, brake lag, and actual braking distance.

The space kept between your vehicle and the one ahead. Air brakes and heavy loads need extra room, and a common guideline is at least one second per ten feet of vehicle length plus one.

The loss of braking power caused by overheating of drums and linings, often from repeated or prolonged braking. It is reduced by braking early and using a lower gear on downgrades.

A device that uses engine compression to help slow the vehicle, reducing reliance on the service brakes. It is commonly used on long downgrades to help prevent brake fade.

A downhill braking technique of applying the brakes firmly to reduce speed, then releasing them to let them cool. It helps control speed on long grades without overheating the brakes.

A hard, controlled stop made when a hazard appears suddenly. The driver applies firm steady pressure and keeps it on rather than pumping the pedal, which wastes air.

A technique of applying the brakes hard, releasing, and applying again repeatedly to slow a vehicle when normal braking is not working. It is used to make use of whatever braking remains.

A loss of braking ability caused by problems such as low air pressure, a major leak, or mechanical damage. The driver should downshift, use the emergency brake, and look for a safe place to stop.

An escape ramp with a gravel or sand bed built on steep downgrades for drivers who lose braking. Steering into it is better than continuing down the grade out of control.