Arkansas CDL Air Brakes Test cheat sheet

The compressor, driven by the engine, builds and maintains air pressure in the system; a belt-driven compressor that fails will cause pressure to fall while driving.

The governor controls when the compressor builds air (cut-in, typically around 100 psi) and when it stops (cut-out, typically around 120-125 psi).

The air compressor is cooled and lubricated by the engine's own cooling and oil systems; it does not have a separate oil supply.

The alcohol evaporator (when equipped) injects alcohol into the air system to prevent moisture from freezing in lines and valves during cold weather.

The safety valve is a spring-loaded valve on the air tank that releases air at a set pressure (commonly about 150 psi) to prevent tank over-pressurization.

The supply reservoir (wet tank) receives air directly from the compressor and is where most moisture and oil drop out; it feeds the primary and secondary circuits.

The application pressure gauge shows how much air pressure is being applied to the brakes, while the supply gauge shows reservoir pressure available for use.

The low air pressure warning device must activate before pressure drops below 60 psi; it may be a red light, a buzzer, or both.

The brake chambers convert air pressure into mechanical force; the pushrod moves the slack adjuster, which rotates the camshaft to apply the foundation brakes.

A dual air brake system has two separate circuits (primary and secondary) so that a failure in one circuit still leaves the other circuit able to stop the vehicle.

In a dual system, the primary circuit typically operates the rear brakes and the secondary circuit typically operates the front brakes.

The check valve in the supply line allows air to flow only toward the reservoirs, so a loss of pressure in one tank cannot drain the other tank.

The double check valve lets air from either the primary or secondary circuit supply a protected device, always admitting air from the higher-pressure side.

The safety valve protects each reservoir from over-pressurization; it is set well above normal governor cut-out and vents excess pressure automatically.

The low pressure warning must activate before pressure falls below 60 psi, and the spring brakes will begin to apply when pressure drops into the 20-45 psi range.

If one circuit fails, the other circuit still functions, but reservoir pressure in the failed circuit may not build and a warning device should activate.

The front brake limiting valve (when equipped) reduces front brake force in slippery conditions; it must be returned to the normal position for full braking.

Check that the air compressor drive belt is tight and not cracked, frayed, or glazed; a loose or broken belt prevents air pressure buildup.

With the engine running and the system at governor cut-out, check for audible air leaks around hoses, fittings, and valves; a leak that is audible is a defect.

Drain all air reservoirs (wet tank, primary, secondary) at the end of the day or during inspection to remove moisture and oil; close the drain valves afterward.

Check that the slack adjusters move freely and that the pushrods have the correct travel; excessive pushrod travel indicates brakes out of adjustment.

Inspect brake drums, shoes, linings, and hoses for cracks, wear, and leaks; oil-soaked linings or damaged hoses are out-of-service defects.

Check the low air pressure warning device by reducing pressure and confirming the warning activates before 60 psi.

Test the spring (parking) brakes by applying them and trying to move the vehicle with the engine at low speed; the vehicle must not move.

Check the safety valve by confirming it is present and not plugged or capped; it must be able to vent excess pressure.

Build air pressure to governor cut-out, then shut off the engine and watch the gauge: pressure should not drop more than about 3 psi in one minute for a single vehicle, or about 4 psi per minute for a combination.

With the engine off and the system fully charged, apply and hold the brakes and check for a pressure drop of no more than about 3 psi per minute; a larger drop indicates a leak.

With the engine running at fast idle, build pressure and confirm the governor cuts out within the normal range (commonly around 120-125 psi).

Pump the brake pedal to reduce pressure and confirm the low pressure warning activates before the gauge falls below 60 psi.

Continue reducing pressure and confirm the spring brakes apply automatically in the 20-45 psi range.

The compressor should build pressure from about 85 to 100 psi within 45 seconds at fast idle in a properly functioning system.

The governor cut-in pressure is typically around 100 psi; the compressor should restart building air when pressure drops to that point.

Before driving, confirm the application pressure gauge responds when the brake pedal is pressed and returns when released.

Air brakes require more stopping distance than hydraulic brakes because of the time needed for air to travel through the lines; allow extra following distance.

Brake firmly and progressively; fanning or pumping the brakes on long downgrades can exhaust air pressure faster than the compressor can rebuild it.

On a long steep downgrade, select a low gear before starting down and use the brakes only to control speed, not to hold the vehicle.

Watch the application pressure gauge and the supply gauge while driving; a falling supply gauge indicates a leak or compressor problem.

Do not ride the brake pedal; continuous light application overheats brakes and can cause fading or failure.

When the brakes get wet, apply them lightly while moving to heat and dry them, but avoid hard stops until braking is normal.

Use the same braking technique for air brakes as for hydraulic brakes on slippery surfaces: gentle, steady pressure and no sudden hard stops.

If the low air pressure warning activates while driving, stop as soon as it is safe and do not continue until the problem is corrected.

If air pressure is lost, the spring brakes apply automatically as pressure falls into the 20-45 psi range, bringing the vehicle to a stop.

If the low pressure warning activates, move to the side of the road, stop, and investigate before continuing; do not try to drive through it.

If one circuit of a dual system fails, the other circuit still provides braking; use it to stop safely and get the system repaired.

The emergency (spring) brakes can be applied by pulling the parking brake control, which releases air from the spring brake chambers.

If the spring brakes apply while driving, the vehicle will stop; steer to a safe location and do not attempt to release them until stopped.

In a runaway situation with failing brakes, use engine braking, downshift, and look for an escape ramp or other safe stopping area.

Do not use the parking brake control as a substitute for the foot brake during normal driving; it is for parking and emergency use.

After any brake failure, the vehicle must be inspected and repaired before it is driven again.

Spring brakes are held applied by heavy springs and released by air pressure; a loss of air pressure causes them to apply automatically.

The parking brake control (usually a yellow diamond-shaped knob) releases air from the spring brake chambers to apply the brakes and admits air to release them.

Never use the parking brake control for a normal stop except in an emergency; it applies the brakes fully and can cause a skid.

Before leaving the vehicle, apply the parking brake and confirm it holds; on a grade, chock the wheels as an added precaution.

Spring brakes cannot be released manually without air pressure unless the chamber has a manual release bolt; follow the manufacturer's procedure.

When air pressure is low, the spring brakes may not fully release; build pressure to normal before moving the vehicle.

The parking brake should hold the vehicle on a grade; if it does not, the brakes need adjustment or repair.

Do not use the parking brake as a substitute for wheel chocks when parking on a steep grade; both may be needed.