CDL Air Brakes test: free practice drill and the numbers to know

The CDL Air Brakes test is the knowledge test you need to drive a commercial vehicle with air brakes: 25 multiple-choice questions in Washington (20 correct to pass) on how the system works, the air brake check, and braking on the road. Without it, your CDL carries an air brake restriction. The free drill below is 12 original questions with every answer explained.

25
questions on the real test: 20
correct to pass (80%): 60 psi
low-air warning, before: 20-45 psi
spring brakes come on

Last checked 20 October 2026 · Counts from the Washington State Department of Licensing; numbers from the state CDL manual, section 5

Free Air Brakes drill: 12 original CDL-style questions at a practice pace of a minute a question, then every answer explained. No sign-up.

12
questions
12 min
time limit
Free
every answer explained

By the LifeScore Editorial Team · Updated 20 October 2026 · How we check facts · How this is scored

Do I need the CDL Air Brakes test?

Yes if you will drive a commercial vehicle with full air brakes, which includes most tractor-trailers and many buses and heavy straight trucks. If you skip it, or take the skills test in a vehicle without air brakes, your CDL is restricted from air-braked vehicles.

Most Class A applicants take it, because almost every tractor-trailer has air brakes. Class B drivers of dump trucks, buses and heavy box trucks usually need it too. Restrictions are expensive to remove later: you retest, and many employers will not hire a driver with one.

The Air Brakes test is shorter than General Knowledge but denser in numbers, and it is where people who skimmed section 5 of the manual most often fail by one or two questions.

What do I need to know about how air brakes work?

That air brakes are three systems in one (service, parking and emergency), that a compressor and governor keep tanks between about 100 and 125 psi, and that spring brakes are held off by air and come on when the air is gone.

The parts the test asks about most
Part	What it does
Air compressor	Pumps air into the storage tanks
Governor	Stops the compressor at cut-out (about 125 psi) and restarts it at cut-in (about 100 psi)
Air tanks	Store enough air to stop several times if the compressor fails; drain daily to remove water and oil
Safety relief valve	Opens if tank pressure gets too high, usually about 150 psi
Brake pedal (foot valve)	Sends air to the brake chambers; the harder you press, the more air
Slack adjusters	Link the push rods to the brake cams; more than about 1 inch of free play means out of adjustment
Spring brakes	Held off by air; come on automatically when pressure falls to between 20 and 45 psi
Low-air warning	Light, buzzer or wig wag that must come on before pressure falls below 60 psi
ABS	Required on air-braked tractors built on or after March 1, 1997 and on other air-braked trucks, buses and trailers built on or after March 1, 1998

Air brakes also have brake lag: air takes time to flow to the chambers, adding about 32 feet to a stop at 55 mph. That is why total stopping distance with air brakes is over 450 feet at highway speed.

What is the air brake check, and what are the limits?

It is a series of tests you do before driving (and on the skills test): pressure build-up, leakage with the brakes released and applied, the low-air warning, the spring brakes, and the parking and service brakes. Each has a number the knowledge test asks about.

The air brake check limits, from the state CDL manual
Test	What should happen
Build-up rate (dual system)	Pressure rises from 85 to 100 psi within 45 seconds at operating rpm
Governor	Compressor cuts out about 125 psi and cuts back in about 100 psi
Released leakage, engine off	Less than 2 psi a minute (single vehicle) or 3 (combination)
Applied leakage, engine off	After the first drop, less than 3 psi a minute (single) or 4 (combination)
Low-air warning	Comes on before pressure falls below 60 psi
Spring brakes	Come on (the knob pops out) between 20 and 45 psi
Parking brake	Holds the vehicle against a gentle pull in low gear
Service brake	Stops the vehicle straight and promptly at about 5 mph

The leakage limits are the most-missed facts on the test. Learn them as a pattern: released is the smaller pair (2 and 3), applied is the larger pair (3 and 4), and a combination always gets one more psi than a single vehicle.

What do CDL Air Brakes questions look like?

Questions on a part of the system, a limit in the brake check or what to do on the road. Three examples follow; the first is worked in full and the others open when you tap.

CDL-style question 1 · Air brake check
With the engine off and the brakes released, a single truck's air pressure drops 3 psi in one minute. Does it pass the static leakage test?
1. AYes; the limit is 4 psi a minute
2. BNo; the limit is less than 2 psi a minute
3. CYes; the limit is less than 3 psi a minute
4. DNo; there should be no drop at all
Worked answer
B: No; the limit is less than 2 psi a minute. Released (static) leakage must be less than 2 psi a minute for a single vehicle and less than 3 for a combination. 3 psi fails on a single truck.
1. Released or applied? Released (brakes off).
2. Single or combination? Single.
3. Limit: less than 2 psi a minute. 3 psi fails.
The trap: Using the combination or applied limit. Four numbers (2, 3, 3, 4) look alike; tie each to its condition.
CDL-style question 2 · Low air
You test the low air pressure warning by fanning the brakes with the engine off. It comes on at 50 psi. What does that mean?
1. AIt is working correctly
2. BIt comes on too late; it must come on before pressure falls below 60 psi
3. CIt comes on too early; it should come on at 20 psi
4. DThe governor is set too high
Show the worked answer
B: It comes on too late; it must come on before pressure falls below 60 psi. The low-air warning must activate before tank pressure falls below 60 psi. At 50 psi it is late, so it needs repair before driving.
1. Recall the rule: the warning must come on before 60 psi.
2. 50 psi is below 60.
3. So the warning is late: have it fixed.
The trap: Confusing the warning with the spring brakes (20 to 45 psi). The warning has to come well before the brakes do.
CDL-style question 3 · Mountain driving
Your safe speed on a long downgrade is 45 mph and you are in the right gear. How should you use the brakes?
1. ALight, steady pressure all the way down
2. BBrake firmly to 40 mph, release, and repeat when you reach 45 mph
3. CBrake to 25 mph, then coast in neutral
4. DFan the pedal quickly every few seconds
Show the worked answer
B: Brake firmly to 40 mph, release, and repeat when you reach 45 mph. Snub braking: brake hard enough to feel a definite slowdown until you are 5 mph below your safe speed, release, and repeat when you are back at the safe speed.
1. Pick the safe speed: 45 mph.
2. Brake until 5 mph below it: 40 mph.
3. Release, let it climb back to 45, and repeat.
The trap: Light, steady braking feels gentle but heats the drums continuously, which causes brake fade.

Get every psi number right

Four Air Brakes drills with fresh numbers each time, and eight full mocks, every answer explained.

The CDL Pass Pack gives you 8 full mocks on the real clock, 12 drills of 12 questions for the sections that cost you points, and a predictor that turns your mocks into the odds of reaching your target. One payment, no subscription, and your free score stays free.

Prep courses for these tests usually cost $19 to $129. The CDL Pass Pack is US$39 (£29), once.

See the CDL Pass Pack How our questions are built and checked

8
timed mocks: 12
topic drills: 1
score predictor

What does the Air Brakes test ask about braking on the road?

How to stop normally and in an emergency, how to go down long grades without brake fade, what to do when the low-air warning comes on, and when not to use the parking brakes.

Downgrades: choose a low gear before the grade, then use snub braking: brake firmly to 5 mph below your safe speed, release, and repeat.
Brake fade: caused by heat. Needing more pressure to hold the same speed is the warning sign. Slow down and stop to let the brakes cool.
Low air warning: stop and park safely as soon as you can, before the spring brakes come on by themselves.
Emergency stops without ABS: controlled braking or stab braking; with ABS, keep steady, firm pressure and steer.
Parking: use the parking brakes, except when the brakes are very hot or wet in freezing weather; then use wheel chocks.

How should I study for the Air Brakes test?

Read section 5 of your state manual twice, make a card of the numbers in the brake check table, then drill until you get the leakage and pressure questions right without thinking.

Section 5 is about twenty pages and almost every page has a testable fact. The questions that cost people the test are rarely about how air brakes work; they are about which number goes with which condition. The drill above changes the numbers on every set, so you have to apply the rule, not recognize an answer.

Once you are above 80% on drills, take the full CDL practice test to check Air Brakes holds up alongside General Knowledge and Combination. If you drive a combination, the Combination test builds on this one with the trailer air system.

Frequently asked questions

How many questions are on the CDL Air Brakes test?

Washington lists 25 questions with 20 correct needed. A few states ask 20; check yours.

What happens if I fail the Air Brakes test?

You can retest it on its own. If you get a CDL without passing it, the license carries an air brake restriction.

At what pressure do spring brakes come on?

Usually between 20 and 45 psi, as the manual says.

When must the low-air warning come on?

Before pressure falls below 60 psi.

What causes brake fade?

Excessive heat from using the brakes too much, often on long downgrades or with brakes out of adjustment.

Pass Air Brakes without a restriction

Four Air Brakes drills, eight full CDL-style mocks and every answer explained, in one payment.