ND CDL Air Brakes cheat sheet

The compressor, driven by the engine, builds air pressure; the governor controls it by cutting the compressor in at about 100 psi and out at about 125 psi on most ND-type systems.

A safety valve (pop-off valve) on the air tank releases pressure at about 150 psi to protect the system if the governor fails; it is a backup, not a normal regulator.

The alcohol evaporator or air dryer removes moisture and oil from the air supply; a saturated dryer cartridge lets water into the tanks and lines and can cause freeze-ups in cold weather.

The supply (wet) tank receives air first and collects moisture; the primary and secondary tanks feed separate circuits (typically front axle versus rear axle) so one failure does not disable all brakes.

A one-way check valve between tanks keeps air from flowing backward, so a leak or failure in one circuit does not drain the other circuit.

The foot valve (treadle valve) meters air to the brake chambers in proportion to pedal travel; it is not an on/off switch, so partial pedal gives partial braking.

The slack adjuster converts pushrod stroke into rotation of the brake camshaft (S-cam brakes); too much slack-adjuster travel means the pushrod stroke exceeds the legal limit and braking force drops.

Spring brakes apply by mechanical spring force when air pressure is released; they are held off by air pressure, so losing air causes them to apply automatically.

The low pressure warning device must activate before pressure drops below 60 psi; on vehicles built after the relevant rule it is a red light plus an audible buzzer.

Static leak test: with the engine off, full pressure, and brakes released, pressure loss must be no more than 2 psi in one minute for a single vehicle and no more than 3 psi in one minute for a combination vehicle.

Applied leak test: with the engine off and the service brake fully applied, pressure loss must be no more than 3 psi in one minute for a single vehicle and no more than 4 psi in one minute for a combination vehicle.

Low pressure warning test: with the engine off, pump the brake pedal to reduce pressure; the warning light and buzzer must come on before pressure falls below 60 psi.

Spring brake (emergency brake) test: after the warning activates, continue to pump the pedal; the spring brakes must fully apply by the time pressure drops to about 20-45 psi.

To test spring brake holding power, park on a grade, release the service brake, and confirm the vehicle does not roll while the spring brakes are set; then reapply air and verify they release.

Build-up test: at fast idle, pressure should rise from 85 to 100 psi within about 45 seconds on a typical system; slow build-up indicates a weak compressor, leaks, or a restricted line.

Compressor cut-in and cut-out: pressure should rise to governor cut-out (about 125 psi) and then hold; a system that never reaches cut-out or constantly cycles has a leak or a faulty governor.

Check the slack adjusters by pulling each one by hand with the brakes released; if a pushrod stroke is beyond the limit or the adjuster moves loosely, the brake needs adjustment or repair.

Drain the air tanks daily to remove moisture and oil; on systems with automatic drains, verify they are functioning, since water in tanks can freeze and block air flow.

Build full air pressure (governor cut-out, about 125 psi) and confirm the low pressure warning and spring brakes work before moving the vehicle.

Allow the engine to idle long enough to build pressure before driving; never drive with the low pressure warning on or with pressure below the minimum operating level.

Use the service brake with steady, progressive pedal pressure; air brakes have a lag between pedal movement and brake application, so avoid pumping or stabbing the pedal.

On long downgrades, select a low gear before starting the descent and use engine braking plus light, steady service brake applications; do not ride the brakes continuously.

Brake fade in air-braked vehicles comes mainly from overheated drums and linings; if fade occurs, reduce speed, downshift, and let the brakes cool rather than increasing pedal force.

Keep pressure above the minimum (typically 100 psi) during normal driving; watch the gauge on grades and at stops because repeated applications use air.

When stopping and parking, set the parking (spring) brakes, place the transmission in a low gear or park, and chock wheels on a grade if required.

In slippery conditions, apply the brakes gently and early; abrupt applications can lock wheels, and a vehicle with spring brakes will apply automatically if pressure is lost.

Release the parking brake only after pressure is up and the vehicle is ready to move; verify the spring brakes have released by checking that the vehicle rolls freely.

If the low pressure warning sounds while driving, stop as soon as it is safe; continued driving will cause the spring brakes to apply automatically at about 20-45 psi.

If air pressure is lost and spring brakes apply, the vehicle will stop; do not attempt to drive through the applied spring brakes, because they cannot be released without air pressure.

If a brake chamber or line fails and air escapes, the one-way check valves isolate the failed circuit; the remaining circuit still provides braking, so brake firmly and stop.

If the service brakes fail but pressure remains, use the parking (spring) brake gradually to slow and stop; a full application can lock the rear wheels and cause a skid.

If the compressor fails or the belt breaks, pressure will not build; stop and do not continue, because the spring brakes will eventually apply on their own.

If a brake is dragging or smoking, stop and let it cool; do not pour water on hot drums, because warping or cracking can result.

If the vehicle begins to skid under braking, release the brake pedal to regain steering control, then reapply with less pressure; air brakes do not have the same feel as hydraulic brakes.

If the low pressure warning and spring brakes activate on a grade, steer to a safe stopping point, set the parking brake, and use wheel chocks before attempting repairs.

After any emergency stop caused by an air system fault, do not resume driving until the fault is found and corrected and full pressure is restored.