OR CDL Air Brakes common mistakes
OR CDL Air Brakes · Understand the traps and how to avoid them.
Describing the low air pressure warning as a light that 'comes on around 60 psi' without stating the activation threshold must be above 60 psi and before the gauge drops below it, and confusing the warning device type (buzzer and/or light).
Candidates memorize a single number from a summary sheet and drop the 'must activate before pressure falls below 60 psi' wording, or they recall only the light and forget the audible warning is also required on vehicles with a warning device.
What to do instead
Learn the rule as: a warning signal (buzzer and/or light) must activate before the air pressure gauge shows 60 psi, giving the driver time to stop safely. Practice stating it as a threshold condition, not a fixed trigger point, and note that the test checks the warning activates with the engine running and the system charged then drained.
Confusing the governor cut-out and cut-in pressures, or stating a single pressure for both, when asked how the compressor is controlled.
The governor is an abstract component and candidates lump it with the safety valve; they remember 'around 120 psi' but not that cut-out is roughly 120-125 psi and cut-in roughly 100 psi, and they cannot explain that the governor unloads the compressor.
What to do instead
Build a small pressure map: compressor builds to cut-out (~120-125 psi), governor unloads the compressor, pressure falls to cut-in (~100 psi), governor loads the compressor again. Tie the governor to its function (unloading the compressor at cut-out) so the numbers attach to a mechanism rather than floating free.
Treating the safety valve as the same device as the governor, or saying the safety valve 'controls' normal system pressure, instead of describing it as a backup that vents at about 150 psi.
Both devices sit on the system and both involve pressure limits, so candidates merge them into one answer and lose the distinction between normal regulation and emergency protection.
What to do instead
Memorize the contrast explicitly: the governor regulates normal compressor cycling; the safety valve is a one-way backup that releases air at about 150 psi if the governor fails. Practice a one-sentence definition of each that names the other as different.
Explaining the dual air brake system as 'two separate systems with two pedals' or as a system where one circuit backs up the other only after total failure, rather than two circuits supplied by one compressor and protected by one-way check valves.
The word 'dual' invites a mental image of two independent trucks, and candidates never connect the check valves to the reason a single leak does not drain both circuits.
What to do instead
State the architecture in order: one compressor and governor feed two circuits; one-way check valves keep each circuit's air from escaping through the other; a failure in one circuit leaves the other with enough pressure to stop. Answer questions about which circuit is protected by naming the check valve, not a second compressor.
Describing the spring brakes as applying when air pressure is high, or forgetting that they are held off by air pressure and apply when pressure is released, including the role of the push rod and the parking brake valve.
The phrase 'spring brakes' suggests springs do the normal braking, and candidates invert the mechanism; they also forget that the same springs are the parking brake and the emergency brake.
What to do instead
Anchor on the failure-safe logic: strong springs apply the brakes; air pressure compresses them to release the brakes. When pressure is lost, the springs apply. Practice explaining that the parking brake valve releases air to apply them, and that this is why a loss of air is a braking event, not a free-roll event.
Performing the static leak test but reporting only 'the gauge did not move' without stating the allowed loss (no more than 3 psi in one minute for a single vehicle, and no more than 4 psi in one minute for a combination) and the correct starting condition (fully charged, engine off, brakes released).
Candidates memorize the procedure steps but not the pass criteria, so they cannot answer a question that asks whether a given pressure drop is acceptable.
What to do instead
Memorize the numbers as a pair: 3 psi per minute single vehicle, 4 psi per minute combination, measured with the system fully charged and the engine off. Practice reading a hypothetical gauge drop (for example, 5 psi in one minute) and deciding pass or fail against those limits.
Skipping or misordering the applied-brake leak test, or confusing it with the static test, so the answer never mentions holding the brake pedal down and watching for additional loss (no more than 3 psi in one minute).
The two leak tests look similar on paper, and candidates collapse them into one step; they also forget that the applied test is done with the pedal held and the system charged.
What to do instead
Separate the tests by what changes: static test with brakes released, applied test with the pedal held down firmly. Learn the applied limit of no more than 3 psi in one minute and the fact that the engine stays off. Practice writing both procedures side by side so the only difference is pedal position.
Answering the low pressure warning test by only listening for the buzzer, without also checking that the gauge is above 60 psi before the test and that pressure is being drained with the engine off, or failing to state what to do if the warning does not activate.
Candidates treat the test as a yes/no listen and skip the setup conditions, so they cannot describe a valid test or the corrective action of not driving the vehicle.
What to do instead
Write the test as a sequence: charge the system above 60 psi, shut off the engine, drain pressure with the brake pedal or drain valve, and confirm the warning activates before the gauge falls below 60 psi. If it does not activate, the vehicle is out of service until repaired. Practice stating both the setup and the consequence.
Describing normal braking as a hard, full application and describing emergency stopping as pumping the brake pedal, instead of explaining that air brakes are applied firmly and held, and that fanning or pumping wastes air.
Candidates carry over habits from hydraulic brakes and from passenger-car advice, and they do not connect pumping to air consumption and the risk of triggering spring brakes.
What to do instead
Practice the rule: apply the brake firmly and hold it; do not pump or fan the pedal because each application uses air and repeated use can drop pressure enough to apply the spring brakes. For a stop from speed, brake early and progressively, and use the same firm-and-hold technique when the system is losing pressure.
Giving a vague answer to 'what do you do if the brakes fail' that jumps straight to the emergency flashers, without naming the ordered actions: downshift, use the parking/spring brake gradually, use engine braking and terrain, and look for an escape route.
Emergency questions feel like common sense, so candidates list one or two actions and miss the graded sequence and the warning against applying the spring brake abruptly at speed, which can lock the wheels.
What to do instead
Memorize a short ordered list: keep steering control, downshift to slow the vehicle, apply the parking brake gradually rather than all at once, use engine braking and the terrain, sound the horn and use flashers, and steer toward an escape route. Practice stating the order and the reason for gradual application (avoiding a skid and loss of control).