CDL Air Brakes Test: The Seven Federal Topics and the Numbers Behind Them

Who takes the CDL Air Brakes test

Federal rules require air brake knowledge from every driver of a commercial vehicle equipped with air brakes (49 CFR 383.111(b)). For licensing, "air brakes" means any braking system that works fully or partly on the air brake principle (383.95(a)(2)). An air-over-hydraulic truck counts.

The pass line is the same as on every knowledge test: at least 80% correct (383.135(a)(1)). If you do not pass the Air Brakes test and the state still issues your permit or license, it must carry an air brake restriction (383.135(a)(2)). Taking the skills test in a vehicle without air brakes leads to the same restriction (383.95(a)). The restrictions guide explains the L and Z codes and how to remove them.

Your state decides how many questions there are and when you take the test. Washington, for example, gives 25 Air Brakes questions and requires 20 correct, which is the federal 80% (as checked on October 28, 2026, per Washington DOL). Other states differ, so check your state's CDL manual. The test format guide covers counts and retests.

The seven federal air brake areas and the questions they produce

The law lists the areas but gives no weights, and the number of questions per area is not public. The middle column below is Passy's reading of what each area usually turns into on a test.

Areas 5 and 6 are where most of the numbers live. Area 3 links this test to the Combination Vehicles test, which also lists air brakes among its topics (383.111(c)(3)(ii)). Full CDL prep planned for launch keeps 207 Air Brakes questions, each with an explanation, in a set of their own, apart from General Knowledge and Combination Vehicles.

For a 2027 test date, nothing above changes. The seven areas, the 80% line and the air brake restriction read the same in the eCFR as of October 26, 2026, and no FMCSA rule published in the Federal Register since mid-2025 amends them.

Air brake numbers to know, with their conditions

These are the typical figures in AAMVA's model manual as printed in Washington's CDL guide (Sections 2.1, 5.1, 5.3, and 5.4), checked against California's commercial driver handbook (DL 650, REV. 12/2019). The manuals call several of them approximate and tell you to check the manufacturer's specifications, and the two states do not print every number the same way. Learn each number with its condition, then confirm it in your own state's manual.

Numbers that come in pairs cause most of the wrong answers here. Full CDL prep planned for launch includes flashcards that drill released versus applied leakage limits, the pair applicants most often swap.

Low-air warning at 60 psi or 55 psi: which number to give

The low-air warning is the figure most likely to differ between sources, so know where each one comes from.

• The federal vehicle rules. Trucks, truck tractors, and buses built on or after March 1, 1975, must have a warning signal that meets Federal Motor Vehicle Safety Standard No. 121 as it applied when they were built (49 CFR 393.51(c)). The current standard requires a warning whenever service reservoir pressure is below 60 psi (49 CFR 571.121, S5.1.5). Power units the standard did not cover must warn at 55 psi and below, or at half the governor cut-out pressure if that is lower (393.51(c)(2)).
• The manuals. Washington's guide, which prints AAMVA's model text, says "before 60 psi" in its air brake chapter (Section 5). Its skills-test chapter, and the AAMVA modernized skills-test supplement Washington now uses, say before 55 psi or the level the manufacturer specifies. California's handbook says the warning comes on between 55 and 75 psi, and its in-cab check says before 55 psi.

For the knowledge test, learn the figure your state's air brake chapter prints. For the skills test, learn the figure your state's skills-test material uses, plus your test vehicle's own specification. In both Washington's and California's skills-test material, an air brake check that is not performed and explained correctly is an automatic failure of the vehicle inspection test. On the road the action is the same whatever the number: stop safely as soon as the warning comes on.

Three gauge-reading scenarios for the Air Brakes test

The drivers below are fictional.

Low air on a city street. Marcus is driving a loaded straight truck when the low-air buzzer sounds and the red light comes on. The needles are falling. The yard is four blocks away, but the right move is to stop safely now, while the service brakes still have air. If he waits, the spring brakes will set on their own somewhere between 20 and 45 psi, and because they do not work on every axle, a loaded truck needs a long distance to stop on them, while a light one on a slick road can skid. The free 20-question CDL set includes a low-air warning item set at highway speed: try the low-air warning question in the free CDL set and compare your reasoning with the explanation for each option.

Water in the tank drain. On a freezing morning, Priya opens the manual drain on an air tank, and a burst of cloudy water comes out before clean air follows. That is the contamination area at work: compressed air carries water and compressor oil, both collect in the tanks, and the water can freeze and stop the brakes from working. She drains each tank completely, as the manual says to do with manual drains at the end of each driving day. An alcohol evaporator lowers the risk of ice but does not replace draining.

A long downgrade. A sign gives a 40 mph safe speed for the grade. Jamal shifts to a low gear before the descent, not partway down. When his speed reaches 40, he brakes just hard enough to feel a definite slowdown, about 3 seconds, until he is near 35, then releases. He repeats this as often as needed. If he finds he needs more and more application pressure to hold the same speed, the brakes are fading, and he should slow down and use a lower gear. The same free set has a downhill braking question, so once the method makes sense, try the downhill braking item in the free mixed CDL set.

Studying for the Air Brakes test after General Knowledge

In the model manual, the Air Brakes material is Section 5, and anyone pulling a trailer also needs the trailer air brake part of Section 6. Start with the parts, because every number above describes one of them, then learn the check, which uses most of the numbers in one sequence. Last, learn the operating practices: controlled and stab braking, antilock brakes, low air, and parking. One tip on parking: the manual warns against setting the parking brakes when the brakes are very hot or very wet in freezing weather, and says to use wheel chocks on a level surface instead.

Learn every number together with its condition: brakes released or applied, single or combination, engine running or off, knowledge chapter or skills-test check. Swapping one half of a pair is an easy way to pick a wrong option. At launch, full CDL prep will let you practice the Air Brakes test on its own or mixed with the other seven tests. When the numbers are solid, walk through the air brake check in order.