FL CDL Air Brakes common mistakes
FL CDL Air Brakes · Understand the traps and how to avoid them.
Confusing the roles of the compressor and the governor, saying the compressor itself cuts out at a set pressure, rather than recognizing the governor is what cycles the compressor between cut-out and cut-in pressures.
Both are described together in study material as the 'charging' part of the system, and candidates blur which component senses reservoir pressure and which physically pumps air.
What to do instead
Anchor the chain: compressor pumps air, governor senses reservoir pressure and controls the compressor's loading (cut-out around 120-125 psi, cut-in around 100 psi), and the safety valve is the independent backup. Practice labeling each component by what it senses versus what it does.
Treating the dual air brake system as two completely separate circuits with no shared parts, instead of recognizing the circuits share the compressor, governor, and supply reservoir but have separate service reservoirs and separate control/brake circuits.
The word 'dual' suggests total independence, and candidates miss that a failure of shared supply components affects both circuits.
What to do instead
Memorize what is shared (compressor, governor, supply tank, lines to the supply tank) versus what is split (two service reservoirs, separate circuits, usually one circuit for front or rear). Explain to yourself why a leak in one service circuit still leaves the other circuit with air.
Skipping the air compressor drive belt or gear check and the reservoir drain test during the pre-trip inspection, focusing only on the gauge readings.
Gauge checks feel like the 'real' air brake test, so candidates jump straight to build-up and leak tests and omit the mechanical and moisture checks.
What to do instead
Use a fixed sequence: belts and mounting, then drain each reservoir to check for excessive water/oil, then build-up time, then static leak test, then applied leak test, then low-pressure warning, then spring brake and parking brake checks. Recite the sequence until it is automatic.
Misstating the low air pressure warning threshold, saying the warning should come on at 60 psi or lower instead of knowing it must activate before pressure drops below roughly 60 psi (commonly tested as 60 psi).
The warning buzzer/light and the spring brake pop-out pressures are close in value and get swapped under exam pressure.
What to do instead
Separate the two numbers: warning device must activate before pressure falls below 60 psi, and the spring brakes are designed to apply when pressure drops into the 20-45 psi range. Write both numbers with their component and rehearse the distinction.
Believing that when air pressure is lost the service brakes will still stop the vehicle, rather than understanding the spring (emergency) brakes apply automatically and the service brakes are lost.
Candidates reason from cars, where brakes are hydraulic and pressure loss is a different failure, and they forget that air pressure holds the spring brakes off.
What to do instead
Trace the logic: air pressure compresses the spring to release the brake; losing air releases the spring and applies the brake. Practice explaining why a vehicle with no air cannot be towed or moved until the spring brakes are mechanically released.
Forgetting to chock the wheels and release the parking brake before performing the static leak test, or performing the test with the engine running and compressor cycling.
Candidates copy a checklist without understanding that the test must be done with a fully charged system, engine off, and brakes released so leaks in the released position are detected.
What to do instead
State the conditions out loud before testing: system fully charged, engine off, wheels chocked, parking brake released, then observe gauge drop over one minute. Then re-apply the brakes and test the applied condition separately.
Mixing up stopping distance factors, claiming speed doubles stopping distance rather than recognizing that braking distance increases with the square of speed under ideal conditions.
Everyday experience suggests a simple proportional relationship, and the squared relationship is counterintuitive without working an example.
What to do instead
Work two numbers: doubling speed from 20 to 40 mph roughly quadruples braking distance (ignoring reaction distance). Also separate total stopping distance into perception, reaction, and braking distance so each piece is clear.
Describing proper braking technique on a long downgrade as continuous light brake application instead of selecting a lower gear and using controlled, intermittent or snub braking to avoid brake fade.
Drivers habitually ride the brakes lightly, and candidates transfer that habit into exam answers.
What to do instead
State the rule: slow down and shift to a lower gear before the grade, use snub braking (apply firmly to reduce speed, then release to cool), and never ride the brakes continuously. Explain that heat buildup causes fade and can lead to loss of braking.
Ignoring the out-of-service criteria for brakes, such as a pushrod stroke beyond the adjustment limit, audible air leaks, or a warning device that fails to activate, and answering that the vehicle can proceed with caution.
Candidates treat these as minor defects because the vehicle still moves, and they do not connect the inspection defect to the out-of-service standard.
What to do instead
Memorize the defect categories that put a vehicle out of service: brake adjustment limit exceeded, air leaks beyond the allowed rate, missing or inoperative warning device, and defective spring brakes. For each, state that the vehicle must not be operated until repaired.
Answering skid-control questions by recommending hard braking and steering away from the skid, rather than releasing the brakes to regain wheel rotation and steering in the direction you want the vehicle to go.
Panic braking feels correct, and the counterintuitive 'steer where you want to go' rule is easy to forget.
What to do instead
Learn the recovery sequence: release the brake to let wheels roll, steer in the direction of the skid (where you want the front of the vehicle to go), and avoid overcorrecting. Connect this to why locked wheels lose steering control.