CDL Class B Practice Test
Class B is the straight truck license. Federally it is Group B, which means a single vehicle rating 26,001 pounds or more, or one of those towing something that rates 10,000 pounds or less. Dump trucks, box trucks, refuse trucks, cement mixers, most city buses. If the load sits on the same frame as the engine, you are almost certainly looking at Class B.
50 questions, 40 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
Class B practice test: quick facts
What this practice test does. Your state's own numbers are cited separately.
- Questions per test
- 50
- Right answers to pass
- 40
- Time limit
- None
- Cost
- FreeNo account, no card, no limit on retries
- Questions in the bank
- 345Drawn 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
- Official state handbooks
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.
- Vehicle inspection 28
- Basic vehicle control 12
- Shifting gears 11
- Seeing 7
- Communicating 11
- Space management 14
- Speed management 15
- Night driving 16
- Extreme driving conditions 19
- Hazard perception 24
- Emergencies 13
- Skid control and recovery 10
- Accidents and fires 13
- Alcohol and drugs 15
- Staying alert and fit to drive 13
- Cargo 20
- Railroad crossings 12
- Rules and disqualification 11
- Getting and keeping a CDL 16
- 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 91
- Moderate 183
- Hard 71
Sample Class B 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 defense, 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.
-
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 50.
More about this test
Fewer tests than Class A, and that is the trap
Class B needs general knowledge, and it needs air brakes on anything air braked. That is usually two written tests rather than the three a Class A driver sits, and the missing one is combination vehicles, because there is no trailer to combine with.
That sounds easier and in one sense it is. But it changes where the risk sits. With only two subjects in play, a weak subject is half your result rather than a third of it. There is nowhere to hide. A Class A candidate who is shaky on air brakes still has two other banks propping up the average. A Class B candidate in the same position is already close to failing.
The air brakes half is the one to worry about
General knowledge rewards experience. If you have driven for years, a lot of it will feel like common sense written down, and it mostly is.
Air brakes does not work that way. It is a numbers test. At what pressure does the low air warning come on. At what pressure do the spring brakes apply on their own. How long the system may take to build back up. How much the pressure is allowed to drop in a minute with the engine off, and how that limit differs between a single vehicle and a combination. None of that is guessable. Drivers who fail Class B usually fail on exactly these questions, and they usually walked in feeling fine.
How to use this
Each run draws fifty questions from both banks at once, weighted the way you would meet them. That matters more here than on any other test on this site, because with two subjects the mix is the whole picture.
At the end you get a score per subject rather than one number. Use it honestly. If general knowledge is at ninety-five and air brakes is at sixty, your overall looks passable and you are not ready. Retry the questions you missed, which is faster than working through everything again, and go back only when both halves are strong.
Every answer cites the handbook and page behind it, with the month it was last checked against that document.
Also worth reading
Last reviewed September 2026. Sources are re-checked against their publishers once a year.
Questions about this test
What does a Class B license let me drive?
Which knowledge tests do I need for Class B?
Can I drive a tractor-trailer on a Class B license?
Do I need air brakes for a Class B bus?
Is Class B easier than Class A?
How long should I study for Class B?
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.
Read up on this first
Drill one subject at a time
Shorter sets on a single part of this test, each with its own explanation and the same cited answers. Useful when one subject is costing you the pass.
- Compressor and governor 4 questions
- Chambers, slack adjusters and drums 4 questions
- Air brake inspection and leak tests 4 questions
- Brake lag and stopping distance 4 questions
- Dual air systems 4 questions
- Using air brakes on the road 5 questions
- Spring and parking brakes 5 questions
- Tanks, valves and lines 4 questions
Other tests to take
- Class A 50 questions, 40 to pass
- Class C 50 questions, 40 to pass
- General Knowledge 50 questions, 40 to pass
- Air Brakes 25 questions, 20 to pass
- Combination Vehicles 20 questions, 16 to pass
- Hazmat 30 questions, 24 to pass
- Tanker 20 questions, 16 to pass
- Doubles/Triples 20 questions, 16 to pass
- Passenger 20 questions, 16 to pass
- School Bus 20 questions, 16 to pass
- Pre-Trip 30 questions
Ready when you are.
50 questions drawn fresh from 345 cited ones. No account, nothing to pay, and every question in the bank is free.