NC CDL Air Brakes cheat sheet

The compressor, driven by the engine, builds air pressure; the governor controls when the compressor pumps (cut-in roughly 100 psi, cut-out roughly 125 psi on most CMVs) and prevents overpressure.

The air dryer removes moisture and oil vapor downstream of the compressor; a failed dryer sends water into the tanks and valves, causing freeze-ups and brake drag in cold weather.

Supply (wet) tank collects the first moisture; dry tanks feed the service brakes. Tank drain cocks must be opened to purge water and oil emulsion.

The foot valve (treadle) is the driver's service brake control: it meters air pressure to the relay valve in proportion to pedal travel, not directly to the brake chambers.

The relay valve speeds application and release by supplying air from a nearby reservoir to the brake chambers; it is why air brakes respond quickly on long wheelbases.

The spring (parking/emergency) brake is held applied by a mechanical spring and released by air pressure; loss of air pressure applies these brakes automatically.

The safety valve on the supply tank protects the system from overpressure (typically set near 150 psi); it is not a substitute for the governor.

The low air pressure warning device must activate before pressure drops below 60 psi: a red warning light and/or buzzer, checked during the pre-trip inspection.

The alcohol evaporator or air dryer injects alcohol into the system to prevent moisture from freezing; the supply tank holds the most water and is drained first.

Test for leaks with the engine off, governor cut-out reached, and the system fully charged: release the parking brake, then watch the gauge for a pressure drop of more than 2 psi in one minute (3 psi in a combination vehicle).

Test the low pressure warning: with the engine off and the key on, fan the brakes to reduce pressure; the warning light and buzzer must come on before the gauge drops below 60 psi.

Test the spring brakes: with the system charged, shut off the engine, fan pressure down, and confirm the parking brake applies when pressure falls to the 20-45 psi range.

Test the service brakes: with the engine running and pressure built, hold the pedal and check for a steady drop; a loss of more than 3 psi per minute indicates a leak.

Check the compressor build rate: with the engine at operating rpm, pressure should rise from 85 to 100 psi within 45 seconds (single vehicle) or 3 minutes (combination).

Drain all air tanks daily: open the drain cocks and let moisture and oil escape until only clean air comes out, then close them.

Inspect the slack adjusters: with the brakes released, mark the pushrod, apply the brakes fully, and measure; a pushrod stroke beyond the manufacturer's limit signals an out-of-adjustment brake.

Check the air compressor belt for cracks, glazing, and proper tension; a slipping belt means slow build-up and possible brake failure on the road.

Verify the air dryer purge valve operates and that the brake chambers, hoses, and fittings show no cracks, chafing, or audible leaks.

Air brakes use three braking systems in one: the service brake (foot valve, modulated), the parking brake (spring applied, air released), and the emergency brake (spring brakes applied by pressure loss).

Brake fade occurs because heat reduces friction between drum and lining; control speed downhill by gearing down and using intermittent braking, not continuous light pedal pressure.

On a long downgrade, select a gear low enough that the engine and retarder hold the speed; apply the service brakes firmly enough to feel them work, then release to let them cool.

Air brake pedal feel differs from hydraulic: a light touch on the treadle meters air, and the pedal travel relates to air pressure delivered, not fluid displacement.

The parking brake control (yellow diamond knob) is pushed in to release and pulled out to apply; it should never be used as a service brake at speed.

Stopping distance increases with air brakes because of lag time: about 0.2 to 0.4 seconds passes before the brakes begin to apply after the pedal is pressed.

When the spring brakes are applied automatically by low pressure, the vehicle stops; do not release the parking brake until pressure is restored above the cut-in point.

Use engine braking and a retarder on downgrades; do not ride the brakes, which overheats drums and can lead to a runaway.

In a combination vehicle, the trailer brakes should apply slightly before or with the tractor brakes; if the trailer pushes the tractor, check the relay valve and brake adjustment.

If the low pressure warning sounds while driving, stop as soon as it is safe: the spring brakes will apply automatically when pressure falls to the 20-45 psi range.

If the spring brakes release slowly or the parking brake will not release, the cause is often low air pressure, a leaking chamber, or a stuck relay valve; do not drive until fixed.

A rapid loss of pressure means a broken line or failed chamber: stop immediately, because the spring brakes will lock the wheels and steering control may be lost.

If the brakes fail on a downgrade, downshift, use the emergency (spring) brake by pulling the parking control, and look for a runaway ramp or escape route.

If a brake chamber or hose fails and air leaks continuously, the compressor may not keep up; the low pressure warning will sound and the spring brakes will engage.

For a frozen brake line in cold weather, use alcohol evaporator or air dryer maintenance; do not pour hot water on lines, which can crack them.

If the parking brake will not release because of a stuck valve, the vehicle cannot be moved safely; call for service rather than forcing the control.

When the spring brakes apply automatically, release the parking control only after pressure rebuilds above the cut-in point; otherwise the brakes will reapply.

A dragging brake (smoking wheel, hot drum) can be caused by a stuck slack adjuster or a failed spring brake chamber; stop and let it cool before continuing.