North Carolina Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the North Carolina Commercial Driver's License knowledge exam. Questions are pulled from a pool of 71 drawn from the AAMVA CDL Manual, which is the source document the North Carolina Division of Motor Vehicles uses to write its actual exam.

How it works: Click an answer. The correct choice highlights in green, and you'll see a short explanation. Aim for 85% or better before you sit for the real test in North Carolina.

Question 1 of 20

In a dual air-brake system, the time required for air pressure to build from 85 to 100 psi should be no more than:

15 seconds
30 seconds
45 seconds
120 seconds

Correct. In dual air-brake systems, air should build from 85 to 100 psi within 45 seconds at engine governed RPM.

Question 2 of 20

The governor controls:

Engine RPM
When the air compressor pumps air into the air storage tanks
Brake-pedal travel
Trailer hand-valve operation

Correct. The governor cycles the compressor: it cuts the compressor in around 100 psi and cuts it out around 125 psi.

Question 3 of 20

For a combination vehicle (tractor and trailer) with engine off and brakes released, the maximum allowable air-loss rate is:

2 psi/min
3 psi/min
4 psi/min
6 psi/min

Correct. A combination vehicle is allowed no more than 3 psi/min with brakes released, or 4 psi/min with brakes applied.

Question 4 of 20

The parts of an air-brake system include:

Air compressor, governor, air storage tanks, foot valve, and brake chambers
Master cylinder, brake fluid, and calipers
Hydraulic pump, lines, and disc rotors
Vacuum booster and reservoir

Correct. Air-brake systems use a compressor, governor, storage tanks, foot valve, and brake chambers — distinct from hydraulic systems used in cars.

Question 5 of 20

The S-cam:

Is a type of fuel pump
Forces the brake shoes against the drum to provide braking action
Adjusts the slack adjuster
Is the parking-brake control

Correct. When the foot valve is pressed, air pushes the brake-chamber pushrod, which moves the slack adjuster, which rotates the S-cam shaft, forcing the shoes against the drum.

Question 6 of 20

Antilock braking systems (ABS):

Replace the regular brakes
Help prevent wheel lockup so you can keep steering during emergency braking
Reduce braking distance under normal conditions
Are not required on combination vehicles

Correct. ABS prevents wheel lockup during hard braking, allowing you to steer to safety. ABS does not necessarily shorten stopping distance.

Question 7 of 20

Slack adjusters need to be checked because:

They wear and require adjustment to maintain braking force
They never need attention
They are decorative
They affect only the parking brake

Correct. Slack adjusters control how far the pushrod must travel to apply the brake. Out-of-adjustment slack adjusters are the leading mechanical cause of out-of-service violations.

Question 8 of 20

A low-air-pressure warning device must come on at or before:

20 psi
40 psi
60 psi
80 psi

Correct. A federal-mandated low-air-pressure warning (light, buzzer, or wig-wag) must activate at or before 60 psi.

Question 9 of 20

When should you NOT use the parking brakes?

When the vehicle is parked
When the brakes are very hot, such as after a long downgrade — they could damage if applied
At any time
When the engine is off

Correct. Hot brakes can be damaged by the spring brakes contracting against hot drums. Let the brakes cool before parking.

Question 10 of 20

The service brakes are operated by:

The yellow parking-brake knob
The red trailer-supply knob
The foot valve (brake pedal)
The hand throttle

Correct. Service brakes are applied by the foot valve / brake pedal. The yellow and red knobs control parking and trailer-supply functions.

Question 11 of 20

The spring brakes:

Are held off by air pressure and apply automatically when pressure drops
Are activated only by the foot valve
Are activated only by the trailer hand valve
Are not used in modern trucks

Correct. Spring brakes use heavy springs that apply the brake when air pressure is released or drops below 20-45 psi — they serve as both parking and emergency brake.

Question 12 of 20

The trailer air-supply control:

Is a yellow diamond knob
Is a red eight-sided (octagonal) knob
Is a green knob
Is the same knob as the parking brake

Correct. The trailer air-supply (tractor protection) is a red eight-sided knob. Push in to supply air to the trailer; pull out to shut off air.

Question 13 of 20

When the spring brakes apply automatically as air pressure drops, this typically happens between:

100 and 120 psi
60 and 80 psi
20 and 45 psi
5 and 10 psi

Correct. Spring brakes will fully apply somewhere between 20 and 45 psi as system pressure drops, depending on the vehicle.

Question 14 of 20

Air brakes are actually three brake systems combined:

Service, parking, and emergency
Engine, parking, and trailer
Drum, disc, and engine
None of the above

Correct. A complete air-brake system has the service brake (foot valve), parking brake (spring/yellow knob), and emergency brake (which is the parking brake working off the spring brake when air drops).

Question 15 of 20

Pushing the brake pedal harder makes:

No difference
More air pressure go to the brake chambers
The compressor run faster
The trailer brakes release

Correct. The foot valve modulates air pressure to the brake chambers in proportion to how hard you press the pedal.

Question 16 of 20

On a vehicle with ABS, in an emergency stop you should:

Pump the brakes
Apply the brakes the way you always would and steer where you need to go
Apply the parking brake
Coast

Correct. ABS lets you brake hard and continue steering. Brake firmly and avoid pumping — pumping defeats the system.

Question 17 of 20

Continuous use of the brakes on a long downgrade can cause:

Improved braking
Brake fade — loss of braking effectiveness due to overheating
No effect
Faster wear of the air compressor

Correct. Holding the brakes overheats them and causes brake fade — drums expand and brake compounds lose effectiveness, reducing braking force.

Question 18 of 20

A long downgrade requires:

Riding the brakes the whole way
Selecting a low gear before starting down and using intermittent firm brake applications
Coasting in neutral
Using the engine brake only

Correct. Pick a low gear before starting down. Use the brakes in firm, intermittent applications: brake to 5 mph below safe speed, release, repeat.

Question 19 of 20

When checking the brakes on a downgrade, you should:

Use the air-pressure gauge to monitor brake-system pressure for excessive use
Pump the pedal
Drive faster
Ignore the gauge

Correct. Watch the air-pressure gauge. If pressure drops faster than the compressor can recover, brakes are being used too aggressively for the gear selection.

Question 20 of 20

Air brakes take longer to work than hydraulic brakes because:

Air takes longer to flow than fluid
Air-brake lines are longer than hydraulic lines and have more components
Drivers wait longer to brake
Air brakes are simpler

Correct. Brake-lag in air systems comes from the time air takes to travel through lines and reach all the brake chambers — typically 0.4 seconds added to perception/reaction time.

About the North Carolina Air Brakes exam

States typically administer 25 Air Brakes questions and require 80% to pass. Questions cover air-brake system parts, dual systems, supply pressure, brake-system warning, slack adjusters, the parking brake, the spring brake, the pre-trip air-system check, and the proper way to perform a leakage-rate test.

The North Carolina Division of Motor Vehicles follows the federal CDL standards established by FMCSA. To earn the Air Brakes credential, you must answer at least 80% of the questions correctly. Many candidates score lower the first time because the test pulls from a large pool — refreshing this page will give you a different mix of questions, drawn from the same authoritative source.

Want more practice? Try the full Air Brakes question bank or browse all North Carolina CDL practice tests.