NE CDL Air Brakes cheat sheet

The compressor, driven by the engine, builds air pressure; the governor controls when the compressor loads (cut-in, about 100 psi) and unloads (cut-out, about 120-125 psi) to keep the system within its operating range.

The air dryer removes moisture and oil vapor from compressed air before it reaches the wet tank; a saturated or failed dryer passes water downstream and causes freeze-ups and corrosion in valves.

Tanks are arranged as wet, primary (rear brake circuit), and secondary (front brake circuit); the wet tank collects condensed water first, and check valves prevent air from flowing backward between circuits.

A safety valve on the wet or supply tank protects against compressor governor failure by venting air at about 150 psi, well above normal cut-out pressure.

The foot valve (treadle) is a graduated control: the driver's pedal position modulates the pressure delivered to the relay valves, so light pedal gives light braking rather than all-or-nothing.

Relay valves are the high-volume valves that send air from the tank to the brake chambers; the foot valve controls them with a small signal pressure, shortening application time on long vehicles.

The slack adjuster converts pushrod linear travel into rotation of the S-cam, which spreads the brake shoes, and it multiplies brake chamber force into drum torque.

Spring (parking) brake chambers contain a strong coil spring that applies the brakes when air is released; charging the chamber with air compresses the spring and releases the brake.

The low-pressure warning switch activates a buzzer and light when system pressure drops below about 60 psi, and it must be checked before each trip.

Test for leaks with the engine off and the system fully charged: release the parking brakes, hold the service brake, and watch the gauge; pressure loss must not exceed 3 psi in one minute for a single vehicle.

For a tractor-trailer combination the leak limit is 4 psi in one minute with brakes released and 6 psi in one minute with brakes applied; larger loss indicates a leak requiring repair before driving.

Check the low-pressure warning by pumping the brake pedal with the engine off; the buzzer and light must come on before pressure falls below 60 psi.

Test the spring brakes by charging the system, then fanning off air with the engine off; the parking brake should fully apply by the time pressure drops to roughly 20-45 psi.

Check the compressor build rate: with the engine at operating rpm, pressure should rise from about 85 to 100 psi within 45 seconds on a single vehicle (within 60 seconds may be allowed for combinations).

Measure pushrod stroke against the vehicle's adjustment limit; a stroke at or beyond the limit, or more than about 1/4 inch of free play at the slack adjuster, means adjustment or repair is needed.

Manual slack adjusters must be adjusted only after confirming the pushrod is fully returned; automatic slack adjusters should not be manually readjusted to hide a stroke problem, since that masks a worn or failing component.

Drain all air tanks daily to remove water and oil; a significant accumulation between drains points to a failing dryer or excessive compressor oil passing.

Check that the governor cuts out at the manufacturer's specified pressure (commonly 120-135 psi) and cuts in again within about 20 psi of cut-out, so the compressor cycles normally.

A steadily falling gauge with the engine running and no brake applied indicates a supply-side leak; a drop only when the brakes are held indicates a service-side leak at a chamber, hose, or valve.

Pressure that will not build, or builds very slowly, points to a worn compressor, a slipping or broken compressor drive belt, a stuck governor, or a major open leak.

Pressure rising above the governor cut-out and venting at the safety valve means the governor or compressor unloader has failed; the vehicle must be taken out of service.

A low-pressure warning that stays on after the gauge reads normal suggests a faulty switch or gauge; the warning system must be treated as a defect until verified.

Brakes that pull to one side, grab, or fade indicate a dragging shoe, oil or grease on a lining, a collapsed hose, or a stuck relay or quick-release valve.

A brake chamber with a ruptured diaphragm or a cut hose will not hold air; on a combination vehicle this can cause a spring brake to apply or a wheel to lose braking entirely.

Continuous air loss audible at a wheel usually means a leaking chamber diaphragm, a cracked hose, or a loose fitting; the vehicle must not be driven until repaired.

Alcohol evaporators or air dryers reduce moisture, but neither fixes a failed compressor; water in the tanks after regular draining indicates the drying system is not working.

If the warning buzzer and light come on while driving, the correct response is to stop safely as soon as possible and not continue until the low pressure is diagnosed.

The parking/emergency spring brakes apply automatically when system pressure falls to roughly 20-45 psi, because spring force is no longer held off by air.

A dual-circuit system splits front and rear (or primary and secondary) circuits so that a failure in one circuit still leaves braking in the other; expect longer stopping distance and a warning.

The tractor protection valve closes and prevents air loss from the tractor if a trailer breaks away or its air line separates, preserving tractor brakes; the trailer emergency line then applies the trailer spring brakes.

The trailer hand valve (trolley valve) applies only the trailer brakes and must never be used for parking; it is intended for testing trailer brakes and checking pushrod stroke.

On a long or steep downgrade, select a gear low enough that the engine and retarder hold the speed, and use the brakes only for short applications rather than continuous light pressure.

Brake fade from overheating is avoided by not riding the brakes; if fade occurs, use engine braking, stop as soon as it is safe, and let the drums cool before continuing.

The stab braking method: apply the brakes firmly until the vehicle slows to about 5 mph below the safe downgrade speed, release fully, and repeat when speed returns.

When the low-pressure warning sounds, downshift, use engine braking, and bring the vehicle to a controlled stop; the spring brakes will apply automatically as pressure continues to fall.

Before descending, test the brakes at low speed to confirm they hold; if the vehicle will not slow or the pedal feels spongy, do not start the downgrade.