North Dakota Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the North Dakota 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 Dakota Department of Transportation Drivers License Division 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 Dakota.

Question 1 of 20

The "tractor protection valve" closes (cuts off air to the trailer) automatically when:

You apply the brakes
Air pressure in the tractor drops to a dangerously low level (typically 20 to 45 psi)
You hit a bump
You shift gears

Correct. The tractor protection valve protects the tractor from a runaway air loss in case of a trailer break-away or air leak — it closes between 20 and 45 psi.

Question 2 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 3 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 4 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 5 of 20

You should drain the air tanks:

Once a month
Once a year
At the end of every working day
Never

Correct. Drain all air tanks at the end of each working day to remove water and compressor oil that has condensed in the tanks.

Question 6 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 7 of 20

The parking-brake control:

Is a yellow diamond-shaped knob
Is a red round knob
Is a green knob
Does not exist on modern trucks

Correct. The parking-brake control is a yellow diamond-shaped knob, distinct from the red round trailer-supply knob.

Question 8 of 20

Manual slack adjusters must be adjusted:

Only by a mechanic at the dealership
By the driver, when needed, following manufacturer specifications
Never
Only at the time of vehicle inspection

Correct. Drivers may adjust manual slack adjusters in compliance with manufacturer specifications. Automatic slack adjusters are now standard but still require periodic checking.

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 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 11 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 12 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 13 of 20

Air tanks must be drained because:

They get heavy
Compressor oil and water can collect inside, contaminating the brake system
They expire
It is required after every trip

Correct. Water and compressor oil collect at the bottom of air tanks. They must be drained to keep the system clean and prevent winter freeze-ups.

Question 14 of 20

The supply pressure gauges show:

The brake-pedal pressure
The pressure in the air storage tanks
The amount of air being used by the brakes
The trailer brake pressure only

Correct. The supply (primary and secondary) pressure gauges show air pressure available for braking — a critical reading before driving and during operation.

Question 15 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 16 of 20

When you make a normal stop with air brakes, you should:

Push the brake pedal down hard
Apply firm steady pressure to slow you smoothly
Pump the brakes
Use only the trailer brake

Correct. For normal stops, apply steady firm pressure on the foot valve and adjust to maintain a smooth stop.

Question 17 of 20

When should you use the trailer hand valve to slow the rig?

Whenever you want trailer-only braking
Never; using the trailer hand valve alone can cause a trailer skid
Always when going downhill
Only on dry pavement

Correct. Using the trailer hand valve alone applies brakes only at the trailer wheels and can cause the trailer to lock up and skid. Always use the foot valve to brake.

Question 18 of 20

A condition called "brake fade" is most likely on:

Long downgrades
Flat highway
Backing into a dock
Fueling

Correct. Brake fade comes from heat from extended use — most commonly on long, steep downgrades where the brakes are working continuously.

Question 19 of 20

Stopping distance for a vehicle with air brakes is the sum of:

Perception distance + reaction distance + brake-lag distance + braking distance
Brake-lag only
Reaction time only
Braking distance only

Correct. Air-brake stopping distance is perception + reaction + brake lag + braking — the brake lag is the unique addition.

Question 20 of 20

You should perform a static air-leakage test by:

Driving and watching the gauge
With the engine off, brakes released, watching air pressure for one minute
With the engine running at high RPM
By tapping the air tank with a wrench

Correct. Engine off, brakes released, fully charged system: watch for air loss over one minute. Then apply brakes and check again.

About the North Dakota 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 Dakota Department of Transportation Drivers License Division 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 Dakota CDL practice tests.