District of Columbia Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the District of Columbia 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 District of Columbia Department 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 District of Columbia.

Question 1 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 2 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 3 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 4 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 5 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 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

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 9 of 20

The application pressure gauge shows:

The pressure in the storage tanks
How much pressure is being applied to the brakes when you press the foot valve
Tire pressure
Engine oil pressure

Correct. The application gauge (when present) shows brake-application pressure — useful for spotting brake-system problems on long downgrades.

Question 10 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 11 of 20

For a single vehicle (not a combination) with engine off and brakes released, the maximum allowable air-loss rate is:

1 psi/min
2 psi/min
3 psi/min
4 psi/min

Correct. A single vehicle is allowed no more than 2 psi/min of air loss with engine off and brakes released.

Question 12 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 13 of 20

In a dual air-brake system, you should let the air pressure build to at least what level before driving?

60 psi
80 psi
100 psi
125 psi

Correct. Wait until the system pressure is at least 100 psi before driving in a dual air-brake system.

Question 14 of 20

For a single vehicle with engine off and brakes applied, the maximum allowable air-loss rate is:

1 psi/min
2 psi/min
3 psi/min
4 psi/min

Correct. A single vehicle is allowed no more than 3 psi/min with brakes fully applied.

Question 15 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.

Question 16 of 20

When the air-pressure protection valve closes during driving (because pressure dropped too low), the trailer:

Continues without effect
Spring brakes apply, and the trailer stops
Loses electrical power
Turns off its lights

Correct. When tractor air pressure falls into the 20-45 psi range, the protection valve closes, cutting trailer air, which causes the trailer spring brakes to apply.

Question 17 of 20

In an emergency stop on a non-ABS vehicle, you should:

Apply the brakes lightly
Use stab braking — full brake until wheels lock, release, repeat
Pump the brakes rapidly
Apply only the trailer hand valve

Correct. Stab braking applies brakes hard until the wheels lock, then releases when you feel skid — this slows the vehicle while keeping it straight.

Question 18 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 19 of 20

When brake drums or shoes get very hot, you should:

Park immediately
Continue driving
Pull over and let them cool, but do not apply the parking brake until they have cooled
Throw water on them

Correct. Park where you can let the brakes cool. Do not apply the parking (spring) brake on overheated brakes — it can damage them or cause warpage.

Question 20 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).

About the District of Columbia 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 District of Columbia Department 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 District of Columbia CDL practice tests.