Georgia CDL Air Brakes Test common mistakes
Georgia CDL Air Brakes Test · Understand the traps and how to avoid them.
When asked what the low pressure warning device must do, the candidate says it should come on at 'about 60 psi' or 'when pressure is low' instead of stating the specific threshold: a warning signal (buzzer and/or light) must activate before the air pressure drops below 60 psi, and on most vehicles it activates around 60 psi.
The candidate memorized the general idea that the warning is tied to low pressure but never anchored the number, and many vehicles warn at slightly different points, so 'about 60' feels safe. Study material often emphasizes the 60 psi figure only in passing while the candidate focuses on the governor cut-out and cut-in values.
What to do instead
Memorize the exact rule as a single sentence: 'The low air pressure warning must come on before pressure falls below 60 psi.' Practice stating it aloud with the number first, then explain that the device is checked during the pre-trip by fanning the brakes down and watching for the warning before the gauge reaches 60 psi.
The candidate describes the static leak test incorrectly, saying 'pump the brakes and see if the gauge drops' without specifying the correct procedure: with the engine off, brakes released, and system fully charged, observe the gauge for one minute and confirm the pressure loss does not exceed the allowed limit (commonly 2 psi for a single vehicle, 3 psi for a combination, and higher limits for the applied-brake test).
The candidate confuses the static leak test with the applied-brake leak test or with the low pressure warning test, and has not practiced the exact sequence of engine-off, fully charged, brakes released, one-minute observation. Text descriptions blur together without hands-on repetition.
What to do instead
Write out the three leak checks as separate numbered steps with their limits: (1) engine off, fully charged, brakes released, one minute, loss must not exceed the vehicle-specific limit; (2) engine off, fully charged, brake pedal fully applied, one minute, loss must not exceed the applied limit; (3) note that a combination vehicle has slightly higher allowances than a single vehicle. Rehearse each as its own answer so the examiner hears the correct test being described.
When asked how to test the spring brakes, the candidate says 'pull the parking brake knob and see if the vehicle holds' but cannot explain that the test is done with air pressure built up, that the spring brakes apply automatically when pressure drops, and that the parking brake valve is pulled out to apply and pushed in to release, with the tractor protection valve and trailer supply line considerations for combination vehicles.
The candidate treats the spring brake test as a simple parking brake check and never learned the relationship between air pressure, the spring brake chambers, and the push-pull valve. The mechanics of the spring brake are abstract, so the candidate skips them.
What to do instead
Learn the cause-and-effect chain: air pressure compresses the spring to release the brake; loss of pressure lets the spring apply the brake. Then state the test: build pressure to governor cut-out, pull the parking brake valve, confirm the brakes hold; on a combination, also test that the trailer spring brakes apply when the supply line is disconnected. Practice explaining why the brakes apply automatically on pressure loss, not just how to pull the knob.
The candidate describes the governor as 'the thing that keeps the pressure steady' and cannot state the cut-out and cut-in pressures, or confuses the governor with the safety valve, the compressor, or the alcohol evaporator.
The governor is an internal engine-mounted component the candidate never sees or touches, so it stays an abstraction. The candidate groups all pressure-regulating parts together and answers with a vague functional description instead of naming the component and its pressures.
What to do instead
Build a component map: compressor (makes air), governor (controls when the compressor builds or stops, typically cutting out around 120-125 psi and cutting in around 100 psi on many vehicles), safety valve (releases air if the governor fails and pressure exceeds the safe limit, often around 150 psi), alcohol evaporator (prevents freezing). For each, state what it does, where it is, and what happens if it fails. Answer with the component name first, then the function.
The candidate explains the dual air brake system as 'two separate brake systems' without describing the split (front axle circuit and rear axle circuit, or tractor and trailer circuits), the fact that each circuit has its own reservoir and check valve, and that a failure in one circuit still leaves the other circuit with braking capability.
The candidate reduces the dual system to a slogan about safety redundancy and never learns the plumbing: supply reservoir, primary and secondary reservoirs, check valves, and the dash gauge showing both circuits. Without the layout, the candidate cannot answer follow-up questions about which circuit feeds which axle.
What to do instead
Draw the system from memory: compressor feeds the supply reservoir through a check valve; the supply reservoir feeds the primary and secondary reservoirs through check valves; the primary circuit typically serves the rear brakes and the secondary the front brakes (or the reverse depending on design), and each has its own low pressure warning. Practice explaining what happens to braking if one circuit loses pressure: the other circuit still provides braking, and the warning alerts the driver.
The candidate answers the pre-trip inspection question by listing checks in a random order, mixing the engine-off checks (leak test, low pressure warning, spring brake test) with the engine-on checks (compressor build-up, governor cut-out, air dryer purge) and skipping the final step of confirming the system builds pressure to the governor cut-out.
The candidate memorized a list of items but not the sequence, and under exam pressure recalls items in the order they happen to remember them. The engine-off versus engine-on distinction is the organizing principle, and without it the answer sounds disorganized even when individual facts are right.
What to do instead
Adopt a fixed sequence and rehearse it: (1) engine off, drain tanks, check the low pressure warning activates before 60 psi; (2) start engine, build pressure, check the compressor builds to governor cut-out and the air dryer purges; (3) engine off, fully charged, brakes released, one-minute leak check; (4) applied-brake leak check; (5) spring brake/parking brake test; (6) check the safety valve and drain valves. State the sequence number before each item so the examiner hears a structured answer.
When asked about braking technique on a long downgrade, the candidate says 'use the brakes gently' or 'downshift' without explaining that the correct technique is to select a lower gear before the descent, use the brakes in short applications to control speed rather than riding them continuously, and avoid overheating that causes brake fade.
The candidate treats the air brake exam as pure component knowledge and does not connect it to driving technique. The phrase 'use the brakes gently' sounds reasonable but is not a technique, and the candidate has not practiced describing a specific downgrade procedure.
What to do instead
Prepare a concrete downgrade answer: before the grade, check the brakes and select a gear low enough that the engine and gearing hold the speed; use the brakes in short, firm applications to bring speed down, then release to let them cool; never ride the brakes continuously; if brakes fade, use the emergency stopping procedure and find a safe place to stop. Practice saying 'short applications, then release' as the core rule.
The candidate cannot describe what to do when the low pressure warning activates while driving, answering only 'pull over' without stating the immediate actions: stop as soon as it is safe, do not continue driving, and understand that as pressure drops the spring brakes will apply automatically when pressure falls to the spring brake application range (often around 20-45 psi, with the parking brake valve popping out on many vehicles).
The candidate learns the low pressure warning as a pre-trip test item only and never rehearses it as an in-motion emergency. The automatic spring brake application is a safety feature the candidate has read about but not connected to the warning scenario.
What to do instead
Rehearse the in-motion scenario as a short script: warning sounds, check the gauge, stop safely and promptly, do not try to drive to the shop, expect the spring brakes to apply automatically as pressure continues to fall, and use the parking brake valve to control the stop if needed. Link the script to the pre-trip test so the candidate sees the same device in both contexts.
The candidate confuses the safety valve with the low pressure warning device, saying the safety valve warns the driver of low pressure, or says the low pressure warning releases air to protect the system.
Both are pressure-related safety devices with similar-sounding names, and the candidate learned them in the same study session without separating their functions. The safety valve protects against overpressure; the warning device alerts the driver to underpressure, and the candidate swaps the two.
What to do instead
Pair them in a comparison table: low pressure warning device = buzzer/light, activates below 60 psi, alerts the driver; safety valve = pressure relief valve, typically set around 150 psi, releases air to the atmosphere to prevent overpressure if the governor fails. State the direction of the problem (too little versus too much pressure) with each device, and practice explaining what each protects and how you would notice a failure of each.
The candidate answers the air dryer question by saying 'it dries the air' but cannot explain that the air dryer removes moisture and oil before air reaches the tanks, that it purges automatically when the compressor reaches cut-out, and that a failed dryer can send moisture into the system and cause brake problems in cold weather.
The air dryer is a maintenance item the candidate rarely inspects personally, so it stays a one-line fact. The candidate does not connect the purge cycle to the governor cut-out, which is the observable event during the pre-trip build-up check.
What to do instead
Learn the dryer as part of the build-up sequence: compressor builds pressure, governor cuts out around 120-125 psi, the dryer purges with an audible burst of air, and moisture is expelled. Then state the consequences of failure: moisture and oil carry into the tanks and valves, can freeze in cold weather, and can cause brake drag or failure. During the pre-trip, listen for the purge at cut-out as confirmation the dryer is cycling.