NM CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system, usually by cycling between a cut-out pressure and a lower cut-in pressure as air is used.

Valve on or near the compressor that senses system pressure and controls whether the compressor pumps or idles, setting the cut-out and cut-in pressures.

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

The lower pressure at which the governor restarts the compressor after air has been used, typically about 20 to 25 psi below cut-out pressure.

Dash instrument showing reservoir pressure in psi; on a vehicle with dual air systems it shows both the primary and secondary circuits so the driver can monitor pressure.

One of two separate compressed-air circuits on a dual system, usually supplying the rear service brakes; a failure in the other circuit should not drain it.

The second independent circuit on a dual air system, usually supplying the front service brakes, kept separate so one circuit failure still leaves braking ability.

Brake design with two separate air circuits fed by the same compressor, providing a backup circuit so a single leak or failure does not eliminate all service braking.

Tank that stores compressed air for a brake circuit; it holds enough volume to allow several brake applications even if the compressor stops working.

Manual or automatic valve at the low point of a reservoir that releases accumulated water and oil, which must be drained regularly to protect the brake system.

Device that injects alcohol vapor into the air system to lower the freezing point of moisture, helping prevent ice from blocking valves and lines in cold weather.

Cartridge or desiccant unit that removes moisture from compressed air before it reaches the reservoirs, reducing corrosion and freeze-ups in the system.

Pressure-relief valve on a reservoir that opens at a set pressure, usually about 150 psi, to protect tanks and components from over-pressurization.

Pedal-operated valve the driver presses to apply service brakes; it meters air pressure to the brake chambers in proportion to how far the pedal is pushed.

Cylinder where air pressure pushes a diaphragm or piston to move a pushrod, converting air pressure into the mechanical force that applies the foundation brakes.

Rod extending from the brake chamber that transfers diaphragm movement to the slack adjuster; its stroke length indicates brake adjustment and chamber condition.

Lever connecting the pushrod to the brake camshaft that converts pushrod travel into rotation of the camshaft and provides a means of adjusting brake clearance.

Slack adjuster that self-adjusts during normal brake use to maintain correct pushrod travel; it still must be checked and can fail to adjust properly.

Distance the pushrod travels when brakes are applied; excessive stroke means brakes are out of adjustment and stopping power is reduced.

Drum brake in which an air-driven camshaft rotates an S-shaped cam that spreads brake shoes against the drum to create friction and slow the wheel.

Rotating metal cylinder attached to the wheel that brake shoes press against; friction between shoes and drum converts motion into heat to stop the vehicle.

Brake chamber section with a strong spring that applies the brakes when air pressure is released, acting as parking brake and emergency stopping force.

Dash valve that releases air from spring brake chambers to apply parking brakes, and supplies air to release them when pushed in with pressure built up.

Valve that closes air lines between tractor and trailer when pressure drops or the trailer is disconnected, sealing each unit's air supply so it can still brake.

Two hoses, typically colored blue for service and red for emergency/supply, that carry air between tractor and trailer; they must be coupled correctly and checked for leaks.

Coupling fittings that connect tractor and trailer air hoses; they must be locked together and checked for leaks and proper seal before driving.

Dash light and buzzer that activate when reservoir pressure falls to about 60 psi or below, warning the driver that braking ability is being lost.

Automatic spring brake application that occurs when system air pressure drops too low, bringing the vehicle to a stop to prevent loss of all braking.

Procedure of building full pressure, shutting off the engine, and watching the gauges to confirm pressure loss stays within allowed limits over a set time.

Measured drop in reservoir pressure per minute during a leak test; excessive loss means the system needs repair before the vehicle is driven.

Check performed with the brakes fully applied and engine off, watching for pressure drop that would indicate leaks in the applied side of the system.

Loss of braking power caused by overheating of drums and shoes from repeated or prolonged braking, common on long downgrades if brakes are not used correctly.

Device that uses engine compression or exhaust restriction to slow the vehicle and reduce reliance on service brakes, helping prevent overheating and fade on downgrades.

Technique of braking firmly to reduce speed by about 5 to 6 mph, then releasing brakes to cool, repeated on downgrades instead of continuous light braking.

Emergency technique of pressing the brake pedal fully and holding it, releasing only if wheels lock, used when brakes must stop the vehicle quickly.

Braking at the maximum pedal pressure that does not lock the wheels, keeping steering control and the shortest possible stopping distance on most surfaces.

Condition in which a braked wheel stops rotating and slides, causing loss of steering control and longer stopping distances, especially on slippery surfaces.

Gauge showing how much air pressure the driver is applying to the brakes, helping judge braking force and detect improper brake adjustment or leaks.

Time needed for the compressor to raise reservoir pressure from a low point to cut-out pressure; long build-up time signals a weak compressor or a leak.