CDL Air Brake Inspection Practice Questions
Brakes that are out of adjustment are the fault found more often than any other at a roadside inspection, and the driver who finds it first is the one who pulled on the slack adjusters that morning. Nothing on an air system announces itself. A leak big enough to empty the tanks halfway down a grade makes no sound at all at idle. A cracked drum stops the vehicle perfectly well right up until the day it does not. There is no warning light for any of it. The only way to know the state of an air system is to measure it against a number and see whether it passes.
4 questions, 3 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 brake inspection and leak tests practice test: quick facts
What this practice test does. Your state's own numbers are cited separately.
- Questions per test
- 4
- Right answers to pass
- 3
- 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 brake inspection and leak tests 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.
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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.
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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 4.
More about this test
That is what makes this subject different from most of what you have to learn for the air brakes test. There is almost nothing to reason out. There are four leak rates, one movement limit, one crack limit and one pressure, and either you know which number belongs to which check or you are guessing.
The four leak rates, and why they get swapped
Two tests, two vehicle types, four numbers. The first test is done with the system fully charged, the engine shut off and the parking brake released, with nothing applied. A single vehicle should lose less than 2 psi in a minute. A combination should lose less than 3 psi in a minute.
The second test starts the same way, but this time you hold the foot brake down for the full minute. A single vehicle may lose 3 psi, a combination 4 psi. Each applied limit is exactly one psi higher than the unapplied limit for the same vehicle.
That neat pattern is also the trap. The 3 psi figure appears in both tests, meaning a combination unapplied and a single vehicle applied, so a candidate who has memorised the numbers as a loose set of four rather than as a grid picks the right digit attached to the wrong test. Learn it as a grid. Single or combination down one side, applied or not applied along the other, and every number one higher as you move right or down.
What the leak test is actually detecting
Holding the brake pedal down puts air into parts of the system that are empty the rest of the time: the service lines, the brake chambers, the relay and quick release valves. That is why the applied limit is more generous. You have opened up more of the system and given it more places to leak from. It is also why doing only the unapplied test tells you less than half of what you need to know, because a chamber diaphragm can be split and you will never see it on a gauge until you press the pedal.
The combination limits are higher for the plainest of reasons. There is more vehicle. More lines, more couplings, more glad hands, so more seals that are each allowed their small share. If a reading fails, it is not a number to average away or drive gently on. It means finding the leak and fixing it before the vehicle moves.
Pulling on the slack adjusters
This one cannot be done from the cab, and it cannot be done safely without setting up first. Park on level ground, chock the wheels, then release the parking brakes. The order matters and the reasoning behind it is the whole point of the question. The spring brakes hold the adjusters rigid, so nothing will move while the parking brakes are on, which means they have to come off. A vehicle with its parking brakes released is a vehicle that can roll, so the chocks and the level ground come first, not after.
Put gloves on and pull hard on each adjuster you can reach. Watch the point where the push rod attaches. More than about one inch of movement there and the brake probably wants adjusting. Too much slack means the push rod runs out of useful stroke, the brake never reaches full force, and the vehicle becomes very hard to stop at exactly the moment you most need it not to be.
Automatic adjusters still have to be checked
Everything built since 1994 has automatic slack adjusters, so unless you are on something genuinely old, this is what you have. Self-adjusting is not the same as self-checking. They take up slack during full brake applications, and they are good at it, which is exactly why an automatic adjuster that has drifted out of adjustment is worth paying attention to. It is not lazy. It is reporting a mechanical fault, either in itself, in the foundation brake behind it, or in the way it was installed.
So winding it back by hand is the wrong answer. The symptom disappears, the fault stays, the adjuster wears out early and the brake goes out of adjustment again shortly afterwards. In the meantime you are driving a vehicle you now believe is fine. Manual adjustment of an automatic adjuster is an emergency stopgap to get somewhere safe, and the vehicle belongs in a repair shop.
Drums, discs and linings
Crack length is measured against the drum, not with a tape. A crack longer than half the width of the friction area is too long. Any crack at all is worth watching, but half the friction width is the line, and candidates who expect an answer in inches go looking for one and find a wrong option waiting for them.
Linings are judged separately and by eye. They must not be loose, must not be soaked in oil or grease, and must not be dangerously thin. Oil on a lining is the one people underrate. It does not look dramatic, and it takes a very large bite out of how much the brake can actually do.
Testing the low pressure warning
Shut the engine off, turn the electrical power on, and fan the brake pedal on and off to bleed pressure down. The signal must appear before pressure drops below 55 psi, read on the lowest tank in a dual system.
The engine being off is the part that gets missed. Leave it running and the compressor simply replaces the air as fast as you let it out, so pressure never falls far enough to prove anything. Electrical power has to be on or the light and the buzzer have nothing to work with. And the reading comes from the lowest of the two gauges, because a warning that only watches the healthier half of a dual system is not a warning at all.
What the check is really proving is that you will get told. The warning is the only notice you get that the system is losing air, and it has to come well before the spring brakes decide the matter for you.
Also worth reading
Last reviewed September 2026. Sources are re-checked against their publishers once a year.
Questions about this test
Do I really have to wait a whole minute for each leak test?
Is this the same thing as the pre-trip inspection test?
Why do I have to release the parking brakes to check slack adjusters?
My truck has automatic slack adjusters, so is this still my job?
What do I do if the low air warning does not come on before 55 psi?
Are these figures the same in every state?
Which of the four leak rates is the one people get wrong most often?
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
- Chambers, slack adjusters and drums 4 questions, 3 to pass
- Compressor and governor 4 questions, 3 to pass
- Dual air systems 4 questions, 3 to pass
- Brake lag and stopping distance 4 questions, 3 to pass
- Spring and parking brakes 5 questions, 4 to pass
- Tanks, valves and lines 4 questions, 3 to pass
- Using air brakes on the road 5 questions, 4 to pass
Ready when you are.
4 questions drawn fresh from 65 cited ones. No account, nothing to pay, and every question in the bank is free.