CDL air brakes practice test
25 questions, and 20 right clears the federal 80 percent bar. Each answer is tied to the rule it tests. No signup, no email.
25 questions, 40 minutes. The timer starts when you begin, and answers are shown at the end.
How the real New York exam is graded
New York DMV grades the written test on two conditions, and a paper has to meet both: at least 14 of the 20 questions right, and at least 2 of the 4 road sign questions right. A score of 14 or more still fails if 3 of the 4 sign questions are wrong. The car exam simulator here holds you to both, drawing exactly 4 road sign questions into every 20 question sitting. Class M uses the same 20 and 14, which is what the motorcycle paper runs at; it has no drawn sign questions to apply the sign condition to, so it is marked on the overall count only. DMV publishes no time limit for the office test and no rule ending it early, so the allowance on each paper is ours.
This paper allows 40 minutes. That allowance is ours, not a federal rule, because no published limit exists for this test.
Every question in this New York cdl air brakes, with answers
All 25 questions this paper draws from, each with the correct answer, why it is correct, and the federal regulation it was drafted against. The paper itself picks a randomized subset and shuffles the options, so reading the list through is review rather than a shortcut.
1. The low air pressure warning must come on before pressure drops below:
- 40 psi
- 60 psi (correct answer)
- 80 psi
- 100 psi
Answer: 60 psi. Federal rules require the warning to activate before air pressure falls below 60 psi. It is your signal to stop somewhere safe while you still have braking, not a caution to press on.
Source: 49 CFR 393.51
2. On a tractor, spring brakes typically come on fully when air pressure falls to somewhere between:
- 50 and 60 psi
- 60 and 80 psi
- 20 and 45 psi (correct answer)
- 10 and 15 psi
Answer: 20 and 45 psi. As pressure keeps falling past the warning, the spring brakes apply, commonly between 20 and 45 psi. They are mechanical: the springs do the work, which is why they still hold when the air is gone.
Source: FMCSA air brake system guidance
3. What holds a parking brake on when the engine is off?
- An electric solenoid
- Hydraulic pressure
- Residual air pressure in the tanks
- Powerful springs, held off by air when driving (correct answer)
Answer: Powerful springs, held off by air when driving. Spring brakes are held OFF by air pressure. Lose the air and the springs apply the brakes. That is the fail-safe: a leak stops the vehicle rather than leaving it with nothing.
Source: FMCSA air brake system guidance
4. The air compressor governor controls:
- When the compressor pumps air into the tanks (correct answer)
- Brake application pressure
- Trailer brake timing
- Engine speed
Answer: When the compressor pumps air into the tanks. The governor cuts the compressor in and out to keep tank pressure within range, commonly cutting out around 125 psi and cutting back in around 100 psi.
Source: FMCSA air brake system guidance
5. Why must air tanks be drained?
- To reduce weight
- To remove water and compressed oil that collect in them (correct answer)
- To reset the governor
- To recalibrate the gauges
Answer: To remove water and compressed oil that collect in them. Compressing air condenses water, and oil carries over from the compressor. Left in the tank, water can freeze and disable the brakes in cold weather. Manually drained tanks should be emptied at the end of each working day.
Source: 49 CFR 393.50; FMCSA maintenance guidance
6. An alcohol evaporator in an air brake system is there to:
- Lubricate the compressor
- Clean the brake linings
- Reduce the risk of ice forming in the air system in cold weather (correct answer)
- Boost braking power
Answer: Reduce the risk of ice forming in the air system in cold weather. It introduces alcohol into the system to cut the risk of ice in valves and lines during cold weather. The level must be checked daily in winter, and draining tanks daily still matters.
Source: FMCSA cold weather operation guidance
7. A dual air brake system means:
- Two compressors
- Air brakes plus hydraulic brakes
- Two brake pedals
- Two separate air systems supplied by one compressor and governor (correct answer)
Answer: Two separate air systems supplied by one compressor and governor. One compressor and governor feed two independent systems, usually one for the front axle and one for the rear. If one fails the other still stops the vehicle, though the distance will be longer.
Source: FMCSA air brake system guidance
8. Before driving a dual air system, air pressure should build from 85 to 100 psi within:
- 45 seconds (correct answer)
- 2 minutes
- 5 minutes
- 15 seconds
Answer: 45 seconds. A dual system should build from 85 to 100 psi within about 45 seconds at operating engine speed. Slower than that suggests the system will not keep up in use.
Source: FMCSA pre-trip inspection guidance
9. With the engine off and brakes released, the maximum air loss for a straight truck is:
- 1 psi per minute
- 2 psi per minute (correct answer)
- 3 psi per minute
- 5 psi per minute
Answer: 2 psi per minute. Released, a single vehicle may lose no more than 2 psi per minute and a combination no more than 3. With the brakes applied the allowances become 3 and 4 respectively.
Source: FMCSA air brake inspection procedure
10. With the engine off and the brakes APPLIED, the maximum air loss for a combination vehicle is:
- 6 psi per minute
- 2 psi per minute
- 3 psi per minute
- 4 psi per minute (correct answer)
Answer: 4 psi per minute. Applied, a combination may lose no more than 4 psi per minute, against 3 for a single vehicle. More than that means a leak that must be repaired before the vehicle moves.
Source: FMCSA air brake inspection procedure
11. Testing the low pressure warning, you should:
- With the engine off, pump the brake pedal to reduce pressure and confirm the warning activates above 60 psi (correct answer)
- Disconnect a line and listen
- Rely on the gauge alone
- Drive until the warning sounds
Answer: With the engine off, pump the brake pedal to reduce pressure and confirm the warning activates above 60 psi. Build pressure, shut off the engine, chock the wheels, then step on and off the pedal to bleed pressure down. The warning must come on before 60 psi. If it does not, the vehicle is unsafe to drive.
Source: FMCSA pre-trip inspection procedure
12. Continuing to pump the brake pedal down after the warning comes on should cause:
- The engine to stall
- The spring brakes to apply, generally between 20 and 45 psi (correct answer)
- The gauge to reset
- Nothing further
Answer: The spring brakes to apply, generally between 20 and 45 psi. This is the second half of the same test. The tractor protection valve and parking brake knob should pop out and the spring brakes apply, confirming the fail-safe works.
Source: FMCSA pre-trip inspection procedure
13. A manual slack adjuster that moves more than about one inch when pulled by hand means:
- The drum is worn
- The brakes are correctly adjusted
- The brakes are out of adjustment and must be corrected (correct answer)
- Air pressure is too high
Answer: The brakes are out of adjustment and must be corrected. Excessive free play means the pushrod is traveling too far before the shoes contact the drum, which lengthens stopping distance. Out-of-adjustment brakes are a leading vehicle factor in commercial crashes.
Source: 49 CFR 393.47; FMCSA inspection guidance
14. Brake lag on an air system adds roughly how much before braking begins?
- About two seconds
- About five seconds
- No delay
- About half a second (correct answer)
Answer: About half a second. Air has to travel and act, adding roughly half a second before the brakes take hold. At 55 mph that is around 30 extra feet on top of perception and reaction distance.
Source: FMCSA air brake operating guidance
15. The proper way to brake down a long steep grade is:
- Select a low gear first, then apply firmly to drop about 5 mph below your safe speed and release (correct answer)
- Pump the pedal rapidly
- Use the trailer hand valve only
- Hold steady light pressure the whole way
Answer: Select a low gear first, then apply firmly to drop about 5 mph below your safe speed and release. Gear first, then use snub braking: apply firmly enough to lose about 5 mph, release, and repeat as speed builds again. Riding the brakes lightly overheats them and causes fade exactly where you need them.
Source: FMCSA mountain driving guidance
16. Brake fade happens because:
- Air pressure drops
- Heat reduces the friction between lining and drum, and heat expands the drum away from the shoe (correct answer)
- The compressor cannot keep up
- The slack adjusters loosen
Answer: Heat reduces the friction between lining and drum, and heat expands the drum away from the shoe. Heat is the enemy. It cuts the friction the lining can generate and expands the drum outward so the shoe has further to travel. Both effects grow the longer the brakes are held on.
Source: FMCSA air brake operating guidance
17. You should NOT use the parking brake when:
- The engine is running
- Parked on a grade
- The brakes are very hot, or wet in freezing conditions (correct answer)
- The vehicle is loaded
Answer: The brakes are very hot, or wet in freezing conditions. Very hot brakes can be damaged by being clamped, and wet brakes in freezing weather can freeze on. Let them cool, or in freezing wet conditions dry them by driving gently in a low gear with light brake pressure.
Source: FMCSA air brake operating guidance
18. The proper braking technique for a normal stop with air brakes is:
- Maximum pressure then release
- Use of the trailer hand valve alone
- Rapid pumping throughout
- Steady, controlled pressure that does not lock the wheels (correct answer)
Answer: Steady, controlled pressure that does not lock the wheels. Apply enough pressure to stop safely and hold it steady, easing off as speed drops. Pumping wastes air and can drop pressure toward the warning point on a long descent.
Source: FMCSA air brake operating guidance
19. The trailer hand valve (trolley valve) should:
- Never be used for parking, and not be used to prevent a jackknife (correct answer)
- Be applied before every stop
- Replace the foot brake in the wet
- Be used for parking
Answer: Never be used for parking, and not be used to prevent a jackknife. It applies trailer brakes only. Using it to park is unsafe because the trailer air can leak away, and using it to correct a skid drags the trailer and makes a jackknife more likely, not less.
Source: FMCSA combination vehicle guidance
20. If the air compressor develops a leak while you are driving, what stops the vehicle?
- The trailer brakes only
- Nothing, you must coast to a stop
- The spring brakes apply as pressure falls (correct answer)
- The engine cuts out
Answer: The spring brakes apply as pressure falls. That is the point of a spring brake. As air is lost the springs progressively apply, so the vehicle stops rather than losing braking entirely. Your job is to get somewhere safe before that happens.
Source: FMCSA air brake system guidance
21. Wet tanks in an air system should be drained:
- Monthly
- Only before an inspection
- Weekly
- At the end of each day of operation, where drains are manual (correct answer)
Answer: At the end of each day of operation, where drains are manual. Manually drained tanks are emptied at the end of each working day. Automatic drain valves do the job continuously, but should still be checked because a failed valve is invisible until the brakes freeze.
Source: FMCSA maintenance guidance
22. Total stopping distance for an air-braked vehicle is:
- Perception plus reaction plus BRAKE LAG plus braking distance (correct answer)
- Braking distance only
- Reaction plus braking distance
- Perception plus reaction plus braking distance
Answer: Perception plus reaction plus BRAKE LAG plus braking distance. Air brakes add a fourth element that hydraulic systems do not have. Leaving brake lag out of the calculation understates the real distance by roughly 30 feet at highway speed.
Source: FMCSA air brake operating guidance
23. A safety relief valve in the first tank protects the system by:
- Warning of low pressure
- Releasing air if pressure gets too high, commonly around 150 psi (correct answer)
- Draining moisture
- Applying the spring brakes
Answer: Releasing air if pressure gets too high, commonly around 150 psi. If the governor fails and the compressor keeps pumping, the relief valve vents air rather than letting the tank over-pressurize. A valve that has released is a fault to be investigated, not reset and forgotten.
Source: FMCSA air brake system guidance
24. Before an air brake test you should always:
- Run the engine at full throttle
- Release the parking brake
- Chock the wheels (correct answer)
- Disconnect the trailer
Answer: Chock the wheels. Several parts of the test involve releasing brakes with the engine off. Chocking the wheels first is what stops the vehicle rolling away while you are working through it.
Source: FMCSA pre-trip inspection procedure
25. Your brakes have got wet driving through deep water. You should:
- Stop and wait for them to dry
- Use the parking brake to dry them
- Brake hard once to dry them
- Apply light brake pressure while driving slowly in low gear to dry them out (correct answer)
Answer: Apply light brake pressure while driving slowly in low gear to dry them out. Gentle pressure while moving slowly generates heat that drives the water off. In freezing conditions this also stops them freezing on, which is why parking wet brakes in the cold is a bad idea.
Source: FMCSA air brake operating guidance
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Original practice questions written from the rules themselves, not the examination questions New York DMV uses. New York DMVneither runs this site nor endorses it.