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

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

A) Safety valve

B) Treadle valve

C) Draining valve

D) Foot valve

2 / 30

Foundation brakes are used

A) only in emergencies.

B) at each wheel.

C) at each axle.

D) when parking.

3 / 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) The length of the vehicle plus 20 feet

B) Just over the length of a football field

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

D) Half the length of a football field

4 / 30

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

A) All of the above

B) Air hoses connected to brake chambers

C) Brake drums or discs

D) Manual slack adjusters on S-cam brakes

5 / 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) 3 psi for both

B) 4 psi for single, 5 psi for combination

C) 2 psi for single, 3 psi for combination

D) 4 psi for both

6 / 30

The main purpose of the alcohol evaporator is to

A) remove alcohol from the compressor.

B) put alcohol in the air system.

C) remove alcohol from the storage tank.

D) put alcohol in gas tanks.

7 / 30

When should you NOT use the parking brakes?

A) When testing the spring brakes

B) If the brakes are wet during freezing weather

C) All of the above

D) If the brakes are very hot

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) Both the primary and secondary systems

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

C) The secondary system only

D) The primary system only

9 / 30

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

A) Governor

B) Air compressor

C) Reservoir

D) Safety valve

10 / 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) All of the above

B) Brake lingings do not heat up as much.

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

D) Air usage is less with light steady pressure.

11 / 30

Spring brakes do which of the following?

A) They are held by mechanical force.

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

C) They use air pressure to stop the vehicle.

D) They will come on fully at 50 psi.

12 / 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) 150

C) 160

D) 140

13 / 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

14 / 30

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

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

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

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

D) one air brake system and one brake control.

15 / 30

Pressing and releasing the brake pedal quickly and repeatedly

A) will help cool your brakes.

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

C) will stop your vehicle faster.

D) pumps air back into the air brake.

16 / 30

Which of the following statements is true?

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

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

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

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

17 / 30

How often should you drain your air tanks?

A) At the end of each workday

B) Twice a week

C) Biweekly

D) Once a week

18 / 30

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

A) 30

B) 50

C) 60

D) 40

19 / 30

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

A) brake lag

B) perceptive

C) None of the above

D) reaction

20 / 30

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

A) Stop-light switch

B) Supply pressure gauge

C) Low-air-pressure warning signal

D) Application pressure gauge

21 / 30

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

A) pull the parking brake control knob.

B) shut the engine off.

C) step on and off the brake pedal.

D) chock the wheels.

22 / 30

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

A) the brakes will stop the vehicle very suddenly.

B) it probably needs adjustment.

C) it has the perfect amount of slack.

D) it will pass roadside inspection.

23 / 30

Air brakes use

A) cold air.

B) hot air.

C) compressed air.

D) decompressed air.

24 / 30

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

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

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

C) Turning the steering wheel while braking

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

25 / 30

The most common foundation brake is the

A) wedge brake.

B) S-cam drum brake.

C) front brake.

D) disc brake.

26 / 30

When braking on downgrades,

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

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) use stab braking until wheels lock, then release.

27 / 30

The braking power of spring brakes depends on

A) All of the above

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

C) proper adjustment.

D) traction

28 / 30

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

A) emergency

B) parking

C) trailer

D) service

29 / 30

Air brake systems are three braking systems combined. They are

A) emergency, parking, service.

B) parking, service, hand.

C) None of the above

D) emergency, service, hand.

30 / 30

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

A) Drive to the nearest rest area.

B) Use stab braking until you stop.

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

D) Nothing; the vehicle will stop automatically.

Your score is

The average score is 82%

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