CDL Air Brakes Practice Test 2026: Practice test 3

Practice test 3 of 3 for the CDL Air Brakes Practice Test: 30 more questions with answers and explanations.

Commercial vehicle
United States
80 questions
Checked October 10, 2026

Question 1 of 30
Which of these is a problem with brake linings?
- Linings that are clean and dry
- Linings soaked with oil or grease
- Linings that are firmly attached
- Linings of normal, even thickness
Show answer
Answer: B. Linings soaked with oil or grease
Linings must not be loose, soaked with oil or grease, or dangerously thin.
Question 2 of 30
How do you test the low pressure warning signal?
- Drain the tanks while driving and see when the light comes on
- Engine off, key on, pump the brake pedal; it must come on before 60 psi
- Run the engine at high rpm until the warning light comes on
- Pull out the trailer air supply knob and watch the gauge
Show answer
Answer: B. Engine off, key on, pump the brake pedal; it must come on before 60 psi
Turn the engine off, turn the key to power, and step on and off the brake to lower pressure. The warning must come on before 60 psi.
Question 3 of 30
During the spring brake test, you keep pumping the brake pedal to lower air pressure. What should happen?
- The parking brake knob should pop out
- The safety valve should open and release air
- The governor should cut out the compressor
- Nothing should happen until 0 psi
Show answer
Answer: A. The parking brake knob should pop out
As pressure falls to the maker's level, the tractor protection valve and parking brake valve should close and pop out.
Question 4 of 30
How do you check the governor's cut-in and cut-out pressures?
- At fast idle, pumping should stop near 125 psi and restart near 100 psi
- Open the safety valve by hand and listen for the compressor
- Drive at 55 mph and read the gauge while braking hard
- Shut the engine off and watch the gauge for one full hour
Show answer
Answer: A. At fast idle, pumping should stop near 125 psi and restart near 100 psi
With the engine at fast idle, the compressor should cut out near 125 psi. Lower the pressure with the brake pedal; it should cut in near 100 psi.
Question 5 of 30
For the air leakage test, how should you set up the vehicle?
- Tanks drained to 60 psi, engine running
- Fully charged, engine off, parking brake released
- Parking brake applied and engine running
- Engine running at high rpm, brakes applied
Show answer
Answer: B. Fully charged, engine off, parking brake released
With a fully charged system (typically 125 psi), turn off the engine, release the parking brake and time the pressure drop.
Question 6 of 30
A single vehicle's brakes are released and the engine is off. What is the most air pressure it should lose in one minute?
- Less than 2 psi
- No loss at all
- Less than 5 psi
- Less than 10 psi
Show answer
Answer: A. Less than 2 psi
With brakes released, a single vehicle should lose less than 2 psi per minute.
Question 7 of 30
A combination vehicle's brakes are released and the engine is off. What is the most air pressure it should lose in one minute?
- Less than 3 psi
- Less than 8 psi
- Less than 15 psi
- Less than 1 psi
Show answer
Answer: A. Less than 3 psi
With brakes released, a combination vehicle should lose less than 3 psi per minute.
Question 8 of 30
With the brakes applied at 90 psi or more, what is the most air a single vehicle should lose per minute after the first drop?
- 10 psi
- 1 psi
- 6 psi
- 3 psi
Show answer
Answer: D. 3 psi
With the brakes applied, the leakage should be no more than 3 psi per minute for a single vehicle.
Question 9 of 30
With the brakes applied at 90 psi or more, what is the most air a combination vehicle should lose per minute after the first drop?
- 12 psi
- 2 psi
- 4 psi
- 8 psi
Show answer
Answer: C. 4 psi
With the brakes applied, a combination vehicle should lose no more than 4 psi per minute.
Question 10 of 30
Your vehicle loses air faster than the allowed leakage rate. What should you do?
- Drain the tanks and drive
- Have the leaks fixed before driving
- Drive slowly to the next stop
- Raise the governor setting
Show answer
Answer: B. Have the leaks fixed before driving
Too much leakage means the system needs repair before the vehicle is driven.
Question 11 of 30
Air pressure builds up too slowly during your inspection. What should you do?
- Do not drive until it is fixed
- Drive and let it build on the road
- Turn off the low air warning
- Pump the brakes to build pressure faster
Show answer
Answer: A. Do not drive until it is fixed
If pressure builds too slowly, the system could fail. Have it fixed before driving.
Question 12 of 30
With a belt-driven air compressor, what should you check in the engine compartment?
- That the belt is loose enough to slip
- That the belt is wet with oil
- That the belt is in good shape and tight
- That the belt is removed for summer
Show answer
Answer: C. That the belt is in good shape and tight
If the compressor is belt-driven, make sure the belt is in good condition and properly tight.
Question 13 of 30
When making a normal stop with a manual transmission, when should you push in the clutch?
- When engine rpm is near idle
- Before you start to brake
- Never during a normal stop
- As soon as you start braking
Show answer
Answer: A. When engine rpm is near idle
Wait until the engine is near idle before pushing in the clutch, then choose a starting gear once stopped.
Question 14 of 30
Using the stab braking method, how long can it take for locked wheels to start rolling again after you release the brakes?
- They start rolling instantly
- About ten seconds
- About five seconds
- Up to one second
Show answer
Answer: D. Up to one second
It can take up to one second for the wheels to roll after you release. If you reapply too soon, the vehicle won't straighten out.
Question 15 of 30
Why does a vehicle with air brakes take longer to stop than one with hydraulic brakes?
- Air brakes are weaker than hydraulic brakes
- Air brakes only work on the rear wheels
- Air takes time to flow to the brakes (brake lag)
- The governor holds back air when you brake
Show answer
Answer: C. Air takes time to flow to the brakes (brake lag)
Air brakes take about half a second or more to work after you press the pedal. This is called brake lag.
Question 16 of 30
At 55 mph on dry pavement, about how much distance does air brake lag add?
- About 100 feet
- About 3 feet
- About 300 feet
- About 32 feet
Show answer
Answer: D. About 32 feet
At 55 mph, the air brake lag adds about 32 feet to stopping distance.
Question 17 of 30
About how far does it take a vehicle with air brakes to stop from 55 mph on dry pavement?
- About 250 feet
- About 1,000 feet
- Over 450 feet
- About 150 feet
Show answer
Answer: C. Over 450 feet
Total stopping distance at 55 mph with air brakes is over 450 feet.
Question 18 of 30
Which four parts make up total stopping distance for a vehicle with air brakes?
- Perception, reaction, coasting and braking distance
- Perception, reaction, brake lag and effective braking distance
- Reaction, shifting, braking and skidding distance
- Following, perception, braking and parking distance
Show answer
Answer: B. Perception, reaction, brake lag and effective braking distance
Air brakes add brake lag to the usual perception, reaction and braking distances.
Question 19 of 30
What causes brake fade?
- Too much air pressure in the brake lines
- Water mixing with the fuel in cold weather
- Cold air building up inside the tanks
- Too much heat from overusing the brakes
Show answer
Answer: D. Too much heat from overusing the brakes
Excess heat reduces friction in the linings and makes the drums expand, so the brakes stop working well.
Question 20 of 30
Some brakes on your vehicle are out of adjustment. How does this lead to brake fade?
- The ABS stops working
- The out-of-adjustment brakes get too cold
- The other brakes do more work, overheat and fade
- The air pressure rises too high
Show answer
Answer: C. The other brakes do more work, overheat and fade
Brakes out of adjustment do less work, so the others overheat. Check adjustment often, because hot brakes can go out of adjustment fast.
Question 21 of 30
You are going down a long grade in the proper low gear. When should you release the brakes?
- When your speed is about 5 mph below your safe speed
- Only at the bottom of the grade
- When you reach 10 mph over your safe speed
- When you are 20 mph below your safe speed
Show answer
Answer: A. When your speed is about 5 mph below your safe speed
Brake just enough to feel a definite slowdown, and release when you are about 5 mph below your safe speed.
Question 22 of 30
On a downgrade, about how long should each brake application last?
- About 30 seconds
- Less than half a second
- About 1 minute
- About 3 seconds
Show answer
Answer: D. About 3 seconds
Each application, from braking to release, should last about three seconds.
Question 23 of 30
Your spring brakes come on because of low air pressure. Why might a heavy load take longer to stop?
- Spring brakes are stronger than service brakes
- Spring brakes only work on the steering axle
- Spring brakes do not work on all axles
- The ABS turns off the spring brakes
Show answer
Answer: C. Spring brakes do not work on all axles
With a heavy load, stopping on spring brakes takes longer because they are not on every axle.
Question 24 of 30
When should you NOT use the parking brakes?
- Whenever you park on a hill or a slope
- When brakes are very hot, or very wet in freezing weather
- Whenever the engine is turned off
- At the end of the working day
Show answer
Answer: B. When brakes are very hot, or very wet in freezing weather
Very hot brakes can be damaged by the parking brakes, and very wet brakes can freeze in place in cold weather.
Question 25 of 30
Your brakes are very hot after a long downgrade and you need to park on a level surface. What should you do?
- Chock the wheels and let the brakes cool first
- Spray the brakes with water to cool them quickly
- Set the parking brake right away to hold the truck
- Leave the engine running in gear to hold the truck
Show answer
Answer: A. Chock the wheels and let the brakes cool first
Do not set the parking brakes on very hot brakes. Chock the wheels and let them cool.
Question 26 of 30
Your brakes are wet and it is freezing outside. How can you keep them from freezing?
- Drain the air tanks before you park the truck
- Brake lightly in a low gear to heat and dry them
- Pump the brake pedal many times while parked
- Set the parking brake firmly as soon as you stop
Show answer
Answer: B. Brake lightly in a low gear to heat and dry them
Use the brakes lightly in a low gear to warm and dry them before parking.
Question 27 of 30
When may you leave a vehicle unattended?
- Any time you will be gone less than 10 minutes
- Only with parking brakes set or wheels chocked
- After turning off the engine, nothing more
- After shifting into neutral and locking the cab
Show answer
Answer: B. Only with parking brakes set or wheels chocked
Never leave a vehicle unattended without applying the parking brakes or chocking the wheels.
Question 28 of 30
Your vehicle has ABS on the tractor only. How should you brake?
- Brake as hard as possible every time you stop
- Brake normally and ease off if the trailer starts to swing
- Use only the trailer hand brake to stop the rig
- Pump the brake pedal very fast in every stop
Show answer
Answer: B. Brake normally and ease off if the trailer starts to swing
Even partial ABS helps. Brake the same way and ease off if the trailer swings or the tractor starts to jackknife.
Question 29 of 30
When may you fully apply the brakes in an emergency stop?
- Only on dry roads without ABS
- When all axles have working ABS
- Never, with air brakes
- Always, on any vehicle
Show answer
Answer: B. When all axles have working ABS
With working ABS on every axle, you can fully apply the brakes in an emergency stop.
Question 30 of 30
What should you check on the air hoses connected to the brake chambers?
- That they are filled with oil
- That they are disconnected
- That they are not cut or worn
- That they are painted red
Show answer
Answer: C. That they are not cut or worn
Check that air hoses to the brake chambers are not cut or worn from rubbing.

Related tests