LA CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system reservoirs so the system always has enough stored air to stop the vehicle.

Control valve on the compressor that starts and stops the compressor based on reservoir pressure, keeping pressure within a set cut-in and cut-out range.

The upper pressure setting at which the governor unloads the compressor so it stops building air, typically around 120 to 125 psi in a commercial vehicle.

The lower pressure setting at which the governor lets the compressor pump again, commonly around 100 psi, so reservoir pressure is restored before it drops too low.

The primary air tank filled directly by the compressor; it feeds the rest of the system and is the tank protected by the low-pressure warning device.

The first tank the compressor discharges into, where moisture and oil condense and can be drained; it is usually fitted with a manual or automatic drain valve.

Valve at the bottom of an air tank used to blow out accumulated water and oil; it must be opened during the pre-trip inspection to check for moisture and contamination.

Cartridge device between the compressor and the wet tank that removes water vapor and oil from compressed air, helping prevent freeze-ups and corrosion inside the brake system.

Device that injects alcohol vapor into the air system to lower the freezing point of moisture, reducing the chance of ice blocking valves in cold weather.

Pressure-relief valve on a reservoir that opens automatically to release air if pressure rises above a safe limit, protecting tanks and lines from over-pressurization.

The brake pedal valve the driver operates; it meters air pressure from the reservoirs to the brake chambers in proportion to how far the pedal is pushed.

Valve mounted near the rear axles that uses a small control signal from the foot valve to send a large volume of air to the brake chambers quickly, shortening lag time.

Valve that exhausts air from a brake chamber close to the axle instead of forcing it back through a long line, speeding up brake release and reducing drag.

Cylinder where air pressure pushes a diaphragm and pushrod to apply the service brake; it converts air pressure into the mechanical force that moves the slack adjuster.

Rod that transfers force from the brake chamber diaphragm to the slack adjuster; its travel length is measured during inspection to judge brake adjustment.

Lever arm that connects the pushrod to the brake camshaft and takes up clearance as linings wear; automatic versions self-adjust, manual versions must be set by hand.

Brake chamber section with a strong spring that applies the brakes when air pressure is released, providing parking and emergency braking if system pressure is lost.

Valve that prevents service brake air pressure and spring brake spring force from applying at the same time on the same chamber, avoiding damage from combined forces.

Audible and visual alert that must activate before pressure drops below about 60 psi, warning the driver that braking force is being lost and a stop is required.

Dash instrument showing air pressure in the system; a driver watches it to confirm the compressor is keeping up and that pressure is not falling while driving.

Check with the engine off and brakes applied in which the driver observes the gauge over time; pressure should not drop more than a few psi per minute.

Leak check made with the brakes fully applied and held; the driver watches for steady pressure loss that would indicate leaks in chambers, lines, or valves.

Escape of compressed air from a line, fitting, valve, or chamber; it shows up as pressure loss, a hissing sound, or a brake that will not hold.

Distance the pushrod moves when the brake is applied; too much travel means the brake is out of adjustment and stopping power is reduced.

Maximum pushrod travel allowed for a given chamber type and size; brakes exceeding it must be adjusted before the vehicle is driven.

The delay between pressing the pedal and the brakes actually applying, caused by the time air takes to travel through the lines; it lengthens stopping distance.

Loss of braking power from overheating, usually after repeated hard or long down-grade braking, where drums and linings get too hot to grip effectively.

Down-grade technique of applying the brakes firmly to slow a set amount, then releasing them to cool, repeating rather than riding the brakes continuously.

Auxiliary slowing device such as an engine brake or exhaust brake that helps control speed on grades without using the service brakes, reducing heat and fade.

Valve on trailers that applies the trailer spring brakes automatically when supply pressure is lost, functioning as an emergency brake system.

Valve that closes off the tractor air supply if a trailer line breaks or separates, keeping enough air in the tractor to stop it.

Coupling that connects the tractor's service and supply air lines to the trailer; it must be sealed and locked so air does not escape.

Check that the compressor raises pressure from about 85 to 100 psi within a set time with the engine at operating speed, showing the system can recover air.

Controlled stop made immediately when the low-pressure warning sounds or pressure keeps falling, using downshifting, retarder, and remaining brake pressure.

Pre-trip checks done from the driver's seat, including gauge readings, low-pressure warning, build-up time, and leak tests before moving the vehicle.

Total distance traveled from perceiving a hazard to a full stop, including perception, reaction, brake lag, and the actual braking distance.