ME CDL Air Brakes cheat sheet

The compressor, driven by the engine, builds and maintains air pressure; a governor cuts it out near 120-125 psi and cuts it back in near 100 psi on most heavy trucks.

Supply (primary) and service (secondary) reservoirs store air; a one-way check valve between them prevents total pressure loss if one circuit fails.

The foot valve (treadle) meters air to the brake chambers; a dual-circuit treadle keeps one circuit operable when the other fails.

The slack adjuster converts brake chamber pushrod stroke into rotation of the S-cam, which spreads the brake shoes against the drum.

An automatic slack adjuster maintains shoe-to-drum clearance; a manual slack adjuster requires periodic adjustment to keep pushrod stroke within limits.

Spring (parking) brake chambers contain a heavy coil spring applied by releasing air; air pressure must be present to compress the spring and release the brakes.

The relay valve speeds application and release at the rear axle by supplying reservoir air directly rather than routing it through the foot valve.

An alcohol evaporator or air dryer removes moisture from the system; a functioning air dryer also filters oil and dirt from compressor discharge.

The low-pressure warning device must activate before pressure drops below 60 psi, and the spring brakes should apply at roughly 20-45 psi on most vehicles.

Static leak test: build to governor cut-out, shut off the engine, and observe gauges; a loss of more than 2 psi in one minute (single vehicle) or 3 psi in one minute (combination) indicates a leak.

Applied leak test: with the engine off and full pressure, hold the foot valve fully applied for one minute; a drop exceeding 3 psi (single) or 4 psi (combination) indicates a leak.

Low-pressure warning test: with the engine off, pump the brake repeatedly and confirm the warning buzzer/light activates before pressure falls below 60 psi.

Spring brake test: continue pumping with the parking brake released; the spring brakes must fully apply by the time pressure drops to about 20-45 psi.

Tug test: with the spring brakes applied and the engine at low idle in a low gear, release the clutch gently; the vehicle must not move if the parking brakes hold.

Brake chamber stroke check: mark the pushrod, fully apply the brakes, and measure travel; a stroke at or beyond the manufacturer's adjustment limit means the brakes are out of adjustment.

Built-up air time: a system at 85 psi should build to 100 psi within about 45 seconds at fast idle on a typical tractor; slower buildup indicates a weak compressor or leaks.

Inspect lines and hoses for cracks, chafing, and contact with moving parts, and check that gladhands and couplers seal without leaks on combination vehicles.

Check that the safety valve on the supply reservoir releases near 150 psi; a valve that never opens or opens early is defective.

Air brakes have a lag: pressure must build in the lines before the brakes apply, so stopping distance is longer than with hydraulic brakes; allow extra following distance.

Brake fade from overheating drums reduces stopping power; use engine braking and a lower gear on long downgrades rather than continuous heavy brake application.

On a long downgrade, select a gear that holds the desired speed without braking, apply brakes firmly only long enough to slow below the safe speed, then release.

Do not fan or pump the brake pedal on a downgrade; each application adds heat and shortens lining life.

The parking brake is a spring brake: set it by pulling the yellow diamond valve, and never set it while the vehicle is moving except in an emergency.

A trailer supply (red) valve must be pushed in to charge the trailer system; pulling it out applies the trailer spring brakes and can be used to test trailer brakes.

Perform a brake check immediately after starting: at low speed, apply the brakes and confirm the vehicle slows and the parking brake holds.

Wet brakes pull to one side or grab; after driving through deep water, apply light pressure to dry the linings while moving slowly.

If the low-pressure warning activates while driving, stop as soon as it is safe; continued driving risks automatic spring brake application and loss of control.

If a brake chamber diaphragm or line fails, the system loses pressure rapidly; the spring brakes apply automatically when pressure falls far enough.

To move a vehicle with a failed air system, the spring brakes can be released mechanically (caging bolt) on many chambers, but the vehicle then has no parking brake.

A compressor failure shows as pressure that will not build; stop and do not continue, because reserve air is limited and will be depleted by repeated applications.

If the brakes fail completely, downshift, use engine braking, and look for an escape ramp or other safe runoff; do not pump the brake in a total failure.

A stuck or leaking relay valve can cause brakes to drag or fail to release; dragging brakes overheat and can cause a fire.

Antilock braking system (ABS) warning lamps that stay on after start indicate a fault; normal braking remains, but wheel lockup protection may be lost.

If air pressure is lost on a combination vehicle, the tractor protection valve closes to retain tractor air and the trailer spring brakes apply.

Manual slack adjusters must be adjusted so pushrod stroke stays within the limits stamped on the chamber or in the manufacturer's specification; over-stroking means readjustment.

Adjust a manual slack adjuster with the brakes released and system pressure at normal: turn the adjusting nut until the linings contact the drum, then back off the specified amount.

Automatic slack adjusters adjust during normal brake applications; a stroke beyond the limit on an automatic slack adjuster signals a mechanical problem, not a need to adjust it manually.

Check brake lining thickness against the manufacturer's minimum; worn linings increase pushrod stroke and reduce braking force.

Drain air reservoir tanks regularly to remove water and oil; in cold weather, moisture freezes and can block lines or valves.

Inspect the air dryer for a saturated desiccant or a failed purge valve; moisture carryover damages valves and can cause freeze-ups.

Check compressor drive belts for tension and wear, and check for oil in the discharge air, which indicates worn compressor rings or seals.

Verify that the governor cut-in and cut-out pressures match specification and that the safety valve is not leaking; a leaking safety valve prevents correct buildup.

ABS prevents wheel lockup during hard braking but does not shorten stopping distance on all surfaces; drivers must still brake firmly and steer.

An ABS malfunction lamp stays on after the vehicle reaches about 5 mph; braking remains but wheel lockup protection may be lost, so reduce speed.

Automatic traction control (ATC) uses the ABS wheel sensors to reduce wheel spin during acceleration; an ATC lamp indicates reduced traction control.

An engine brake (Jake brake) is a compression-release retarder; it is prohibited in some areas and can cause wheel slip on slippery surfaces, so use with care.

A driveline retarder slows the vehicle through the drivetrain; it does not replace service brakes for a full stop and may be restricted on slippery roads.

A front brake limiting valve (older vehicles) reduces front braking force in slippery conditions; modern vehicles typically do not use one.

A trailer hand valve (Johnson bar) applies only the trailer brakes and must never be used for parking; it can cause a jackknife if used while moving.

The air brake endorsement (for example, a restriction-free CDL) requires passing the air brake knowledge test and the pre-trip inspection; a vehicle with air brakes requires the endorsement.

A CDL holder must pass the air brake knowledge test and demonstrate a pre-trip inspection of the air brake system to remove the air brake restriction from the license.

Drivers must perform and pass a brake inspection, including the leak tests, before operating a vehicle requiring air brakes.

A vehicle with an out-of-service brake defect, such as a pushrod stroke beyond the limit or a leaking chamber, must be placed out of service until repaired.

Federal rules require the low-pressure warning device to operate before pressure drops below 60 psi and require spring brakes to apply automatically at low pressure.

The parking brake must hold the vehicle on a grade; a vehicle that rolls with the spring brakes applied is out of service.

Drivers must carry and present the CDL with the correct endorsements and restrictions, and must not operate a vehicle that exceeds the license's restrictions.

Brake system defects found during a pre-trip inspection must be corrected or reported before the vehicle is operated; driving with known defects violates safety regulations.

Hours-of-service and vehicle inspection rules apply to commercial vehicles; a driver must note brake defects on the inspection report and ensure they are repaired.