CDL Air Brakes Knowledge Test 2025

CDL Air Brakes Knowledge Test 2025: Try our free Commercial Driver’s License (CDL) Endorsements Air Brakes Permit Test. There are 30 sample questions and answers for better CDL written exam preparation.

Air brakes use compressed air to make the brakes work. You can apply all the braking force you need to each of the wheels of a heavy vehicle, even units pulling two or three trailers. Air brakes are a safe way of stopping large vehicles if the brakes are well-maintained and used properly. However, you must know more about air brakes than you need to know about the simpler brake systems used on light vehicles.

CDL Air Brakes Knowledge Test 2025

Test Name	CDL Practice Test 2025
Language	English
License	Endorsements Permit
Total Questions	30 MCQs
Test For	Commercial Driver License
Test Preparation	Air Brake Endorsements
Country	USA
Answers	Yes

0 votes, 0 avg

CDL Air Brakes Knowledge Test

CDL Air Brakes Knowledge Test

1 / 30

Air brakes use

A) compressed air.

B) cold air.

C) decompressed air.

D) hot air.

2 / 30

During walkaround inspections of your vehicle, which parts of the air brake system should be checked?

A) Manual slack adjusters on S-cam brakes

B) All of the above

C) Air hoses connected to brake chambers

D) Brake drums or discs

3 / 30

Most newer heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has

A) two separate air brake systems and a single set of brake controls.

B) one air brake system and two sets of brake controls.

C) two separate air brake systems and two sets of brake controls.

D) one air brake system and one brake control.

4 / 30

When should you NOT use the parking brakes?

A) If the brakes are very hot

B) When testing the spring brakes

C) If the brakes are wet during freezing weather

D) All of the above

5 / 30

A safety relief valve is installed in the first tank the air compressor pumps air to. The valve is usually set to open at what psi level?

A) 130

B) 140

C) 160

D) 150

6 / 30

Which of the following tells you how much pressure there is in the air tanks?

A) Supply pressure gauge

B) Application pressure gauge

C) Low-air-pressure warning signal

D) Stop-light switch

7 / 30

Spring brakes do which of the following?

A) They use air pressure to stop the vehicle.

B) They will come on fully at 50 psi.

C) They are held by mechanical force.

D) They will not work if all the air leaks out of the air brake system.

8 / 30

Before driving a vehicle with a dual air system, allow time for the air compressor to build up to 100 psi pressure in which of the following?

A) The secondary system only

B) It only has to build to 60 psi pressure in each system.

C) Both the primary and secondary systems

D) The primary system only

9 / 30

The brake system that applies and releases the brakes when the driver uses the brake pedal is the _____________ brake system.

A) parking

B) emergency

C) service

D) trailer

10 / 30

Air brake systems are three braking systems combined. They are

A) None of the above

B) parking, service, hand.

C) emergency, parking, service.

D) emergency, service, hand.

11 / 30

Which of the following is a proper technique in using controlled braking?

A) Pressing harder on the brakes when you feel the wheels sliding

B) Putting the brakes on very hard to lock the wheels

C) Turning the steering wheel while braking

D) Releasing the brakes if you need to make large steering adjustments

12 / 30

What should you do if the low-air-pressure warning comes on?

A) Drive to the nearest rest area.

B) Stop your vehicle as quickly as possible and safely park it.

C) Use stab braking until you stop.

D) Nothing; the vehicle will stop automatically.

13 / 30

The low-air-pressure warning light must come on before the air pressure in the tanks falls below what psi level?

A) 50

B) 30

C) 60

D) 40

14 / 30

Foundation brakes are used

A) at each axle.

B) when parking.

C) at each wheel.

D) only in emergencies.

15 / 30

In checking that the spring brakes come on automatically, you should not

A) shut the engine off.

B) step on and off the brake pedal.

C) chock the wheels.

D) pull the parking brake control knob.

16 / 30

If your truck or bus has dual parking control valves, you can use pressure from a separate tank to:

A) keep on about your business

B) brake harder if the main tank is getting low

C) release the spring emergency/parking brakes to move a short distance

D) balance the service brake system when you are parked

17 / 30

Which of the following pumps air into the air storage tanks?

A) Air compressor

B) Safety valve

C) Reservoir

D) Governor

18 / 30

The most common foundation brake is the

A) S-cam drum brake.

B) wedge brake.

C) front brake.

D) disc brake.

19 / 30

ou are driving a vehicle at 55 mph on dry pavement under good traction and brake conditions. What is the approximate total stopping distance needed to bring your vehicle to a complete stop?

A) Half the length of a football field

B) The length of the vehicle plus 20 feet

C) Two-and-a-half-times the length of the vehicle

D) Just over the length of a football field

20 / 30

During the walkaround inspection, if a slack adjuster moves slightly more than an inch where the push rod attaches to it, then

A) it has the perfect amount of slack.

B) the brakes will stop the vehicle very suddenly.

C) it probably needs adjustment.

D) it will pass roadside inspection.

21 / 30

How often should you drain your air tanks?

A) Twice a week

B) At the end of each workday

C) Once a week

D) Biweekly

22 / 30

Which of the following statements is true?

A) It is not necessary to use the parking brake or chock your wheels if you are parked on a flat road surface.

B) If you park right after coming down a very steep grade, you should always use the parking brake.

C) If you are gone only a short time, you do not need to use the parking brakes or chock the wheels.

D) Never leave your vehicle unattended without applying the parking brakes or chocking the wheels.

23 / 30

The main purpose of the alcohol evaporator is to

A) put alcohol in gas tanks.

B) remove alcohol from the compressor.

C) put alcohol in the air system.

D) remove alcohol from the storage tank.

24 / 30

You have a fully-charged air system (125 psi). After turning off the engine and releasing the service brake, what are the maximum leakage rates per minute for both single and combination vehicles?

A) 4 psi for single, 5 psi for combination

B) 2 psi for single, 3 psi for combination

C) 3 psi for both

D) 4 psi for both

25 / 30

Total stopping distance for air brakes is longer than that for hydraulic brakes due to ________ distance

A) brake lag

B) None of the above

C) reaction

D) perceptive

26 / 30

When braking on downgrades,

A) use stab braking until wheels lock, then release.

B) go slow enough so that a light, steady use of the brakes keeps you from speeding up.

C) coast downhill, then apply the brakes hard.

D) brake hard, then let up to cool brakes.

27 / 30

The braking power of spring brakes depends on

A) All of the above

B) traction

C) proper adjustment.

D) how far the modulating control lever (if present) is moved.

28 / 30

Pressing and releasing the brake pedal quickly and repeatedly

A) will help cool your brakes.

B) pumps air back into the air brake.

C) will stop your vehicle faster.

D) can let the air out faster than the compressor can replace it.

29 / 30

What protects the tank and the rest of the air system from too much pressure?

A) Treadle valve

B) Safety valve

C) Foot valve

D) Draining valve

30 / 30

Why, on long downhill grades, do experts recommend using a low gear and light steady pressure instead of on-again, off-again braking?

A) Brake lingings do not heat up as much.

B) All of the above

C) Air usage is less with light steady pressure.

D) You can keep vehicle speed constant in a low gear with light steady pressure.

Your score is

The average score is 82%

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