NE CDL Air Brakes key terms

A belt- or gear-driven pump that draws in outside air and compresses it to build and maintain the vehicle's air supply, typically governed to unload around 120-125 psi.

A device that monitors air tank pressure and controls the compressor, letting it pump when pressure drops to the cut-in point and unloading it at the cut-out point to prevent overpressurization.

The maximum tank pressure at which the governor stops the compressor from pumping, usually around 120 to 125 psi in a properly adjusted system.

The lower tank pressure at which the governor restarts the compressor to rebuild air supply, commonly around 100 psi in a properly functioning system.

Storage tanks that hold compressed air for braking and auxiliary systems; they collect moisture and must be drained regularly, and on dual systems they are divided into separate circuits.

A manually operated valve at the bottom of each air tank used to release accumulated water and oil; tanks should be drained daily to prevent corrosion and freeze-ups.

A device that removes moisture and contaminants from compressed air before it enters the tanks, typically using a desiccant cartridge and purge cycle to keep the system dry.

An optional device that injects alcohol vapor into the air system to prevent moisture from freezing in lines and valves during cold weather; it supplements, but does not replace, daily draining.

A pressure-relief valve on the air tank that opens automatically at a preset pressure (usually around 150 psi) to prevent tank rupture if the governor or compressor malfunctions.

The brake pedal assembly that the driver presses to apply air pressure to the brake chambers; it is designed so that partial pedal pressure produces a corresponding partial brake application.

A cylinder containing a diaphragm and pushrod that converts air pressure into mechanical force to apply the foundation brakes when air is admitted.

The rod extending from the brake chamber that transfers force to the slack adjuster; its travel length is a key inspection measurement indicating brake adjustment.

A lever connecting the pushrod to the brake camshaft that converts linear pushrod motion into rotary motion and provides a means to adjust brake clearance.

The distance the brake chamber pushrod moves during a full brake application; excessive travel indicates brakes are out of adjustment and must be corrected before driving.

A procedure performed during the pre-trip inspection to verify that pushrod travel is within the allowed limit, ensuring brakes will apply fully and legally.

A valve that speeds up brake application and release by sending air directly from a nearby tank to the brake chambers when signaled by the foot valve.

A valve that rapidly exhausts air from brake chambers when the brake pedal is released, reducing brake lag and speeding up release for better control.

The delay between pressing the brake pedal and the brakes actually applying, caused by the time air takes to travel through lines; typically about 0.3 to 0.5 seconds.

The total distance a vehicle travels from the moment a hazard is recognized until it stops, including perception, reaction, brake lag, and actual braking distance.

A fail-safe brake chamber design in which a heavy spring applies the brakes when air pressure is released, used for parking and automatically for emergency stopping if pressure is lost.

The dash control that releases air from the spring brakes to apply them for parking; pulling the knob applies the brakes, and pushing it releases them when air pressure is sufficient.

A valve that automatically closes air lines to the trailer if a trailer breakaway or line rupture occurs, preserving the tractor's air supply and applying the trailer spring brakes.

The two hoses (service and emergency/supply) connecting tractor and trailer air systems; the emergency line supplies air to charge the trailer and triggers trailer brakes if disconnected.

The coupling fittings used to connect tractor and trailer air hoses; they must be securely locked and checked for leaks during inspection.

A braking system with two separate air circuits (primary and secondary) so that a failure in one circuit still leaves the other circuit able to stop the vehicle.

A backup system that automatically applies spring brakes or allows controlled stopping if service brake air pressure is lost, often activated by the yellow parking brake knob.

A low-pressure warning signal (buzzer and/or light) that activates when air pressure drops below a safe level, typically around 60 psi, alerting the driver to stop.

A pre-trip check in which the driver builds air pressure, turns off the engine, and repeatedly applies the brakes to confirm the warning activates and spring brakes apply at the proper low-pressure point.

A procedure to check for air leaks by building full pressure, shutting off the engine, and observing pressure gauges over time; excessive drop indicates a leak needing repair.

A dash instrument showing the air pressure in the brake system; dual systems have separate gauges or a dual-needle gauge for primary and secondary circuits.

The temporary reduction or loss of braking power caused by overheating of brake drums and linings, often from excessive or prolonged braking on downgrades.

The use of engine compression or a retarder to slow the vehicle on downgrades, reducing reliance on service brakes and helping prevent brake fade and overheating.

A technique of applying the brakes firmly to reduce speed by about 5 mph, then releasing, repeated as needed on downgrades to control speed without overheating brakes.

The space maintained behind the vehicle ahead; because of brake lag and longer stopping distances, air-braked vehicles need at least one second per 10 feet of vehicle length plus extra for speed.

A required walk-around check before driving that includes testing air pressure build-up, warning devices, brake adjustment, and leaks to ensure the air brake system is safe.