Arkansas CDL Air Brakes Test key terms

An engine-driven pump that draws in outside air and compresses it to build up the air pressure stored in the vehicle's air tanks. It cycles on and off as pressure falls and rises.

A valve that watches tank pressure and turns the compressor's pumping action on when pressure drops too low and off when the tanks reach the proper maximum. It sets the cut-in and cut-out points.

The upper pressure setting at which the governor stops the compressor from pumping, typically around 120 to 125 psi in a properly adjusted system. Pressure should not climb far beyond this point.

The lower pressure setting at which the governor restarts the compressor, commonly about 100 psi. When a driver hears the compressor kick back in, the system has dropped to this point.

A storage vessel that holds compressed air so the brake system has a ready supply even when the compressor is not pumping. Tanks collect moisture and must be drained regularly.

A device mounted between the compressor and the storage tanks that removes water vapor and oil from the compressed air. Keeping moisture out helps prevent freeze-ups and internal corrosion.

A small valve, usually at the bottom of an air tank, used to release accumulated water and sludge. Tanks should be drained daily so moisture does not reach brake components.

An optional device that injects alcohol vapor into the air system to lower the freezing point of moisture. It helps prevent ice from blocking valves in cold weather.

A pressure-relief valve on the air tank that opens automatically if pressure rises above a safe limit, usually around 150 psi. It protects the system from over-pressurization if the governor fails.

A one-way valve that allows air to flow in only one direction, preventing it from escaping backward. Check valves keep air from draining out of the tanks when the compressor stops or a line fails.

A brake system with two separate air circuits, typically one for the front axle and one for the rear, each with its own tank. If one circuit loses pressure, the other still provides braking.

The air circuit that typically operates the rear (service) brakes. It has its own reservoir so a failure in the other circuit does not leave the vehicle without rear braking.

The air circuit that typically operates the front brakes, supplied by its own separate reservoir. It provides a backup source of braking if the primary circuit fails.

A light and often a buzzer that activates when air pressure in the system falls below a safe level, generally around 60 psi or lower. It warns the driver to stop before braking ability is lost.

Brake chambers that use a powerful spring to apply the brakes when air pressure is released. They set automatically when pressure drops too low, acting as an emergency and parking brake.

A brake that holds a parked vehicle in place, usually applied by spring force when air pressure is released. In air-equipped vehicles it is typically controlled by a dash valve.

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

A braking method used when the service brakes fail or pressure is lost, often relying on spring brakes that apply automatically. It can also be applied manually with the parking brake control.

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

The foot valve that the driver presses to send air from the tanks to the brake chambers. The amount of pressure applied depends on how far the pedal is pushed.

A cylinder where air pressure pushes a piston or diaphragm to apply force to the brake mechanism. It converts air pressure into the mechanical force that applies the brakes.

The linkage between the brake chamber push rod and the brake shoes that adjusts for wear. Proper adjustment is critical so the push rod stroke stays within safe limits.

The distance the brake chamber push rod travels when the brakes are applied. Too much stroke means the brakes are out of adjustment and may not apply fully.

The condition of the brake linkage such that the push rod stroke stays within the manufacturer's limit. Brakes out of adjustment reduce stopping power and can fail an inspection.

A systematic check of the air brake system before driving, including draining tanks, checking for leaks, and testing the low pressure warning and spring brakes. It confirms the system is safe to operate.

A check in which the system is charged to normal pressure, the engine is shut off, and the driver watches the gauge to see how much pressure is lost over time. Excessive loss indicates a leak.

A dash instrument that shows the air pressure in each circuit of the brake system. The driver must watch it while driving to catch pressure loss before it becomes dangerous.

A check that the compressor can raise pressure from a low point to the normal operating range within a reasonable time. Slow build-up can signal a weak compressor or a leak.

A check performed with the brakes applied and the system charged, measuring push rod travel to confirm the brakes are within adjustment limits. It is part of the standard brake inspection.

A loss of braking power caused by overheating of the brake drums and shoes, often from prolonged or hard braking. Fading brakes may require much more pedal force and distance to stop.

A downhill braking technique in which the driver applies the brakes firmly to slow the vehicle, then releases them to let them cool before applying again. It helps prevent brake fade on long grades.

A device such as an engine brake or exhaust brake that helps slow the vehicle without using the service brakes. It reduces heat buildup on long downgrades when used properly.

The total distance a vehicle travels from the moment a hazard is seen until it comes to a complete stop. It includes perception, reaction, and braking distance and grows with speed and weight.

A hard, immediate application of the brakes to avoid a collision or respond to a sudden hazard. In an air brake vehicle the driver should press firmly and hold steady pressure.

The actions a driver takes when brakes fail or pressure is lost, such as downshifting, using the parking brake gradually, and seeking a safe place to stop. Panic and abrupt moves should be avoided.

A dangerous condition in which the service brakes and the spring brakes are applied at the same time, creating excessive force that can damage brake parts. Drivers should avoid applying both together.

An electronic system that prevents wheels from locking during hard braking by rapidly adjusting brake pressure. It helps the driver keep steering control but does not shorten stopping distance on all surfaces.