OK CDL Air Brakes cheat sheet

Compressor (engine-driven, gear or belt) supplies air; governor cuts out near 120-125 psi and cuts in near 100 psi on most systems.

Air dryer removes moisture and oil vapor after the compressor; the purge cycle at cut-out expels collected water through the drain valve.

Reservoirs: wet tank receives compressor air first; dry tanks feed the brake circuits; drain cocks are at the low point of each tank.

Foot valve (treadle valve) meters air pressure to the circuit; it is not a hydraulic master cylinder and does not contain brake fluid.

Brake chamber converts air pressure to mechanical force via a diaphragm and pushrod; slack adjuster multiplies that force to the S-cam.

S-cam rotation spreads the brake shoes against the drum; drum brakes self-energize in the applied direction of rotation.

Safety valve on the wet tank opens near 150 psi to protect the system if the governor fails.

Low-pressure warning must activate before pressure drops below 60 psi; on most vehicles a buzzer and light both activate.

Spring brakes apply by stored spring force when air pressure is released; they require 90-100 psi or more to fully release.

Two separate circuits (primary and secondary) each have their own reservoir, check valve, and lines; a leak in one circuit does not drain the other.

Check valves allow air into each tank but prevent backflow, so a failed circuit cannot bleed the other circuit's reservoir.

The foot valve is split or has two separate supply ports so that loss of one circuit still leaves braking on the other.

A properly functioning dual system loses pressure slowly enough in one circuit to allow a controlled stop; the low-pressure warning activates.

The tractor protection valve closes the tractor-to-trailer air lines when tractor pressure falls to about 20-45 psi, protecting tractor brakes.

The trailer hand valve (Johnson bar) applies trailer brakes only; it must not be used for parking because it can leak off.

The front-brake limiting valve reduces front braking force in the bobtail (unladen tractor-only) condition to reduce lockup risk.

The alcohol evaporator or air dryer injects alcohol or dries air to prevent winter freeze-up in lines and valves.

The parking (spring) brake control is a yellow diamond knob; pulling it releases air and applies the springs.

Test air pressure buildup: with engine at operating rpm, pressure should rise from 85 to 100 psi within 45 seconds on a typical tractor-trailer.

Static leak test: with engine off and full pressure, release the service brake and watch for a pressure drop of no more than 3 psi in one minute (single vehicle).

Applied leak test: fully apply the foot valve and hold; leakage must not exceed 4 psi in one minute for a single vehicle or 6 psi for a combination.

Low-pressure warning test: pump the foot valve to reduce pressure; buzzer and light must activate before pressure falls below 60 psi.

Spring brake test: with pressure below 60 psi, the parking brake knob should pop out and the spring brakes should apply.

Test the service brakes at low speed (about 5 mph) in a clear area; the vehicle must stop without pulling to one side.

Check slack adjuster pushrod travel against the manufacturer's limit; excessive travel indicates misadjustment or worn components.

Inspect lines and hoses for cracks, chafing, and leaks; check that glad hands are sealed with good coupler gaskets.

Drain all reservoirs at the end of the day; moisture or oil in the drained air signals compressor or dryer problems.

Continuous low-pressure buzzer plus red warning light below 60 psi means stop and find the leak before driving.

Pressure falling faster than 3 psi per minute (unapplied) or 4 psi per minute (applied) indicates a leak requiring repair.

A compressor that cannot build pressure above 85-100 psi indicates a worn compressor, slipping belt, or stuck governor.

Oil or milky fluid in reservoir drains points to compressor oil passing or a failed air dryer; both damage valves and seals.

Spring brakes that will not release usually reflect pressure below roughly 90 psi, a stuck relay valve, or a kinked line.

Brakes that pull to one side, grab, or fade indicate contaminated linings, a stuck S-cam, or a seized slack adjuster.

A frozen line or valve in winter causes delayed or no brake application; alcohol evaporators and drained tanks reduce risk.

A failed governor that will not cut out causes the safety valve to vent near 150 psi; the compressor keeps running.

Stopping distance grows with speed squared; at 60 mph the air-brake-equipped vehicle needs far more distance than at 30 mph.

Brake fade is heat-induced loss of friction; continuous light application on a long downgrade overheats drums and shoes.

Select a safe gear before starting down a grade; use engine braking and a steady controlled speed rather than riding the brakes.

Snub braking: apply firmly to reduce speed about 5-10 mph, release, and let the brakes cool between applications.

Do not shift to neutral or coast downhill; you lose engine braking and retarder assistance.

Perception distance plus reaction distance plus braking distance equals total stopping distance; reaction distance grows with speed.

Wet brakes: after driving through deep water, apply light pressure to dry the linings before relying on them.

The posted truck speed on a downgrade and advisory signs reflect grade severity; a lower gear is required when the sign warns of steep grades.

Use a retarder or engine brake on grades where allowed; a retarder does not replace the service brakes for final stopping.

If service brakes fail, downshift progressively and use engine braking; apply the parking (spring) brake gradually, not abruptly.

Pulling the yellow parking knob at speed can lock the rear wheels and cause a jackknife; apply it in a controlled manner.

If one circuit fails in a dual system, the other circuit still provides braking; stop and do not continue the trip.

If a trailer breaks away, the emergency (supply line) glad hand disconnects and the trailer spring brakes apply automatically.

If pressure is lost entirely, spring brakes apply when pressure drops to about 20-45 psi; steer to a safe stop.

Use escape ramps (gravel beds) on steep downgrades when brakes fail; steer straight in and expect a sudden stop.

Do not pump the brake pedal in an air system; pumping wastes air and can accelerate pressure loss.

If brakes drag or smoke, stop, chock the wheels, and let the drums cool; never pour water on hot drums.

After any brake failure event, the vehicle must be inspected and repaired before it returns to service.

Federal CDL rules (49 CFR Part 383) require an air brake knowledge test and an air brake restriction (L) if the driver does not pass or the vehicle has no air brakes.

The air brake restriction L prohibits operating a CMV with air brakes; it is removed only by passing the air brake knowledge and skills tests.

A driver must hold a CDL with the proper class (A, B, or C) plus any required endorsements for the vehicle combination.

Federal rules require a pre-trip inspection and a brake system check before each trip; OK follows the federal standard.

Oklahoma adopts the federal CDL standards; Oklahoma-issued CDLs carry the same air brake restriction code on the license.

Drivers must meet the medical certification and self-certification requirements under 49 CFR 391 before operating a CMV.

Periodic inspection (annual) under 49 CFR 396 is required; brake system defects are out-of-service conditions under the CVSA criteria.

Drivers must keep a current driver qualification file and report convictions and suspensions to the employer within the required time.

Hours-of-service limits apply to air-brake CMV operations; fatigue increases stopping distance and reaction time.