Alabama CDL Air Brakes cheat sheet

The compressor, driven by the engine, builds and maintains air pressure; a governor controls when the compressor loads (builds) and unloads (stops building).

The air compressor governor cut-out pressure is typically around 125 psi and cut-in is around 100 psi on most vehicles.

The air dryer removes moisture and oil vapor from compressed air before it reaches the reservoirs, and it purges on the unload cycle.

Supply (wet) reservoir is the first tank after the compressor and collects moisture; it feeds the primary and secondary circuits through a one-way check valve.

The foot valve (brake valve) controls air delivery to the brake chambers in proportion to pedal travel; a dual foot valve has two separate delivery circuits.

The slack adjuster converts pushrod linear motion into rotary motion to apply the foundation brake; automatic slack adjusters maintain pushrod stroke automatically.

Spring (parking/emergency) brake chambers use a heavy coil spring to apply the brake when air pressure is released, and air pressure compresses the spring to release the brake.

The safety valve on the supply reservoir protects the system by venting air if pressure rises above a set limit (about 150 psi).

A dual air brake system has two separate circuits (primary and secondary), each with its own reservoir, so a failure in one circuit does not disable the other.

The primary circuit typically operates the rear brakes; the secondary circuit typically operates the front brakes.

A dual foot valve has two independent delivery sections; loss of air in one circuit still allows the other circuit to deliver air and apply its brakes.

A one-way check valve between reservoirs prevents air from flowing back from a charged circuit into a failed or leaking circuit.

A pressure differential (low-pressure) warning switch activates a dash lamp and buzzer when pressure in either circuit drops below a set level (about 60 psi).

The spring brakes apply automatically when system air pressure falls below the spring brake release threshold, providing emergency stopping capability.

A failure in one circuit reduces total braking capability but does not cause total brake loss because the other circuit remains functional.

Reservoirs in a dual system must be drained separately; each circuit's drain valve is checked during inspection.

Check the slack adjuster pushrod stroke against the vehicle manufacturer's limit; excessive stroke indicates the brake needs adjustment or service.

Inspect brake chambers for cracks, dents, and damage, and check that clamp bands and mounting bolts are secure.

Inspect all air lines and hoses for cracks, chafing, and leaks; check that lines are properly secured and not rubbing.

Drain all reservoirs (supply and each circuit) and check for water, oil, or excessive contamination in the expelled air.

Verify the air compressor builds pressure at the required rate and that the governor cuts out and cuts in at proper pressures.

Check that the low-pressure warning device (lamp and buzzer) activates before pressure drops below about 60 psi.

Test the spring brakes: with the parking brake released and pressure built, they should hold; with pressure released, they should apply.

Check the safety valve, drain valves, and check valves for proper function and freedom from leaks.

Static leak test: build pressure to governor cut-out, shut off the engine, and observe the gauge; a pressure drop of more than 3 psi in one minute (single vehicle) or 4 psi (combination) indicates a leak.

Applied leak test: with the brakes fully applied and engine off, pressure loss should not exceed about 3 psi per minute for a single vehicle or 4 psi for a combination.

Low-pressure warning test: with the engine off, pump the brake pedal to reduce pressure; the warning lamp and buzzer must activate before pressure drops below about 60 psi.

Spring brake test: with pressure built and parking brake released, the vehicle should not move; with air released, spring brakes should apply and hold the vehicle.

Compressor build-up test: with the engine at operating RPM, pressure should rise from about 85 to 100 psi within 45 seconds (typical standard).

Governor cut-out and cut-in pressures should be verified against the manufacturer's specification, commonly about 125 psi cut-out and 100 psi cut-in.

Check that the air dryer purges properly during the compressor unload cycle to confirm moisture removal is working.

Perform a visual inspection of brake adjustment and components before each trip, in addition to the pressure tests.

Air brakes require a longer stopping distance than hydraulic brakes because of lag time in the system; increase following distance accordingly.

Use the brakes in a controlled, steady manner; fanning or pumping the brakes can deplete air pressure and cause brake fade.

On long downgrades, select a low gear and use engine braking to control speed, applying brakes sparingly to avoid overheating and air loss.

Do not ride the brake pedal; continuous application overheats the brakes and can cause fade or failure.

With a dual system, if one circuit fails, the remaining circuit still provides braking; reduce speed and stop safely to inspect the system.

If the low-pressure warning activates, stop as soon as safely possible; the spring brakes will apply automatically when pressure is too low.

Before descending a grade, check that air pressure is at the governor cut-out level and that the system is leak-free.

When parking, set the parking (spring) brakes and use wheel chocks; do not rely on air pressure alone to hold the vehicle.

A rapid loss of air pressure causes the spring brakes to apply automatically, bringing the vehicle to a stop.

If the low-pressure warning activates while driving, downshift, use engine braking, and bring the vehicle to a controlled stop; do not continue driving.

A failed compressor or governor can cause pressure to stop building; watch for slow pressure build-up or failure to reach cut-out.

A leaking brake chamber or line causes continuous air loss; identify and isolate the leak, and do not drive until repaired.

If spring brakes fail to release due to insufficient pressure, the vehicle cannot be moved until air pressure is restored or the brakes are manually caged (released).

Caging a spring brake involves mechanically compressing the spring with a caging bolt to release the brake for towing or repair; it must be removed before returning to service.

Water or oil contamination in the reservoirs can freeze or damage valves; drain reservoirs regularly and maintain the air dryer.

If brakes overheat and fade on a downgrade, use a lower gear and engine braking, and stop to let brakes cool; do not continue at speed.