CDL Air Brakes Practice Test
The air brakes test is the one that quietly decides what work you can take. Skip it, or fail it, and you still get a commercial licence: what you get with it is an air brake restriction, and a restricted licence rules out almost every tractor-trailer, bus and heavy straight truck on the road. Nobody tells you that at the counter, and plenty of people find out after they have already accepted a job.
25 questions, 20 right to pass. Free, untimed, no account.
Correct · A
At roughly what air tank pressure does the governor stop the compressor from pumping any more air?
- About 125 psi Correct answer
- About 100 psi
- About 150 psi
- About 55 psi
Texas Commercial Motor Vehicle Drivers Handbook, 5.1.2 Air Compressor Governor, p. 5-1
Air Brakes practice test: quick facts
What this practice test does. Your state's own numbers are cited separately.
- Questions per test
- 25
- Right answers to pass
- 20
- Time limit
- None
- Cost
- FreeNo account, no card, no limit on retries
- Questions in the bank
- 65Drawn fresh every time you start
- Every answer cited
- YesDocument, section and page on each one
- Last reviewed
- September 2026Re-checked against the source once a year
- Written from
- Texas Commercial Motor Vehicle Drivers Handbook
What this test covers
Every subject below has cited questions behind it. The number is how many are in the bank right now, and each draw is weighted across them.
- Air brake systems 65
How hard the bank is
Every test you start pulls a mix, so you get the easy marks and the ones people actually fail on.
- Straightforward 15
- Moderate 35
- Hard 15
Sample Air Brakes questions
Real questions from the bank, with the answer and the page it came from. Open a few, then take the test properly.
At roughly what air tank pressure does the governor stop the compressor from pumping any more air?
- About 125 psi
- About 100 psi
- About 150 psi
- About 55 psi
Cut-out is around 125 psi. That is where the governor takes the compressor off load and the needle stops climbing. 100 psi is the cut-in figure, 150 psi is where the safety relief valve opens, and 55 psi is the low air warning. Confusing cut-out with cut-in is the classic mistake.
Tank pressure has been dropping as the brakes are used. At about what pressure will the governor let the compressor start pumping again?
- About 100 psi
- About 125 psi
- About 85 psi
- About 55 psi
Cut-in is around 100 psi, cut-out around 125 psi, so the system normally cycles between those two figures. 85 psi is the bottom of the pressure build-up test range and 55 psi is where the low air warning must already have come on.
An air compressor has its own oil supply rather than being lubricated by engine oil. What does that add to the driver's routine?
- Check the compressor oil level before driving
- Drain the compressor at the end of each day of driving
- Top the compressor up with alcohol in cold weather
- Nothing, because the engine oil check already covers it
A compressor with its own oil supply has to have that level checked before driving. Draining at the end of the day is the air tank routine, and the alcohol top-up belongs to the alcohol evaporator, not the compressor.
A governor is not cutting the compressor in and out at the pressures the manufacturer specifies. Why does that have to be fixed before the vehicle goes out?
- It may not keep enough air pressure in the tanks for safe driving
- It stops the brake lights from coming on when the pedal is pushed
- It disables the low air pressure warning signal
- It makes the spring brakes release on their own
The governor is what keeps a working reserve in the tanks. If it cuts in and out at the wrong pressures the reserve may never be there when it is needed. The stop light switch and the low air warning are separate devices, and nothing about a faulty governor releases spring brakes.
The air tanks on a truck have manually operated drain valves. How often does the driver have to drain them?
- At the end of each day of driving
- Once a week
- Before the start of each trip, but not afterwards
- Only when the low air pressure warning comes on
Manual drain valves are the driver's job at the end of every driving day, by a quarter turn or a pull cable, and the tank has to be drained completely. Only automatic drain valves take that job away, and even then the alcohol evaporator still needs checking in cold weather.
Why does the water that collects in the bottom of an air tank matter most in cold weather?
- It can freeze and cause the brakes to fail
- It thins the compressor oil so the compressor seizes
- It sets off the low air pressure warning while driving
- It makes the safety relief valve open below its setting
Compressed air carries water and a little compressor oil, both of which settle in the bottom of the tank. In freezing weather that water can turn to ice and take the brakes out altogether, which is why the tank is drained completely rather than just cracked open.
At about what pressure is the safety relief valve on the first air tank usually set to open?
- 150 psi
- 125 psi
- 100 psi
- 55 psi
The safety relief valve sits in the first tank the compressor pumps to and is usually set for 150 psi, well above the governor's 125 psi cut-out. If it is venting, pressure has gone somewhere it should never reach.
Air is escaping from the safety relief valve on the first air tank. What should the driver do about it?
- Treat it as a fault and have a mechanic repair it
- Nothing, since that is how the valve holds pressure steady
- Drain the tank down and carry on driving
- Reset the governor cut-out setting to a lower pressure
The safety valve is a last line of defence, not a regulator. Air coming out of it means something in the system is letting pressure rise too far, and that is a mechanic's job. Draining the tank or moving the governor setting hides the fault instead of fixing it.
How to use this test
-
Take it cold, once
Do not revise first. You want to know which subjects are actually weak, and a clean run tells you that in ten minutes.
-
Read the reason, not the score
The explanation for each answer opens on the next page with its handbook page. That is the bit that makes it stick.
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Retry only what you missed
The result page has a button for exactly that. Far faster than sitting the whole thing again.
-
Come back on another day
A different draw each time exposes different gaps. Two runs a week beats ten in one evening.
Why this one is different
You can check every answer
Document, section and page on all of them, plus the date it was read. Nothing goes live without all four.
Nothing is behind a paywall
Every question in the bank is free, every time. There is no premium tier holding the hard ones back.
Scored by subject
You find out which topic is costing you marks, not just a number out of 25.
More about this test
Why this test is different
General knowledge asks you to think like a driver. Air brakes asks you to think like a mechanic for twenty minutes. You need to know what a governor does, why there are two separate air systems and what happens when one of them fails, what the spring brakes are for, and why a wet tank drains water that was never poured into it. It is a system built out of parts you cannot see working, and the test is written by people who know most candidates have only ever read about it.
The parts that catch people out
Almost everybody who fails this test fails on the same handful of things. The order of the seven-step air brake check, because it is long and easy to scramble under pressure. The pressure at which the low air warning must come on, and the lower pressure at which the spring brakes apply on their own. What a leak rate test proves, and what counts as too much loss in a minute. Whether you pump the brakes on a long downgrade or hold them, and why the answer is not what feels natural. And the difference between brake fade and brake lag, which sound alike and are nothing alike.
Then there is the one nobody expects: what to do when the air pressure is dropping and you cannot stop it. That question appears in some form on most versions of this test, and a guess is usually the wrong way round.
How to use this practice test
Twenty-five questions are drawn fresh from the bank each time you start. Answer one, and the next page opens with the reason it was right or wrong and the handbook section and page it came from. That matters more here than on any other test, because air brake answers turn on exact figures, and a figure without a source is just a rumour someone typed on the internet.
At the end you get your score split by subject rather than one number. If you are strong on the inspection routine and weak on the dual system, that is one evening of reading, not a whole re-study. The retry button takes only the questions you got wrong.
What it will not do
It is not your state's test and nobody outside the licensing agency holds that question pool. It also is not a substitute for standing in front of a real vehicle with somebody who can point at the parts. Do both. This drills the material until the numbers stick; the yard teaches you what they look like.
Also worth reading
Last reviewed September 2026. Sources are re-checked against their publishers once a year.
Questions about this test
Do I have to take the air brakes test?
What happens if I fail the air brakes test?
How many questions are on the real air brakes test?
Is air brakes harder than general knowledge?
Do I need to know the seven-step air brake check word for word?
Does this test cover air over hydraulic brakes?
Still not sure?
Every question names the document and page behind it, so a wrong answer is something you can prove rather than argue about.
Other tests to take
Ready when you are.
25 questions drawn fresh from 65 cited ones. No account, nothing to pay, and every question in the bank is free.