Oklahoma Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the Oklahoma 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 Service Oklahoma Driver License Services 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 Oklahoma.

Question 1 of 20

Air storage tanks are used to:

Carry coolant
Hold compressed air for use by the brakes
Store fuel
House the alternator

Correct. Air storage (or "supply") tanks hold compressed air ready for brake application. Trucks have multiple tanks for redundancy.

Question 2 of 20

Which of these is the safest speed for descending a long, steep grade?

The posted speed limit
A speed slow enough that the brakes can hold without overheating, typically below the speed limit
Whatever speed you went up the hill
Maximum gear-down speed

Correct. Choose a speed and gear that lets the brakes work intermittently without continuous pressure. Posted truck-grade signs help.

Question 3 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 4 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 5 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 6 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.

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

After connecting the trailer air lines, you should:

Drive away
Charge the trailer air system, check that the trailer brakes work, and inspect glad-hand connections
Lock the trailer brakes
Turn off the supply

Correct. Charge the trailer system, then test the trailer brakes by pulling out the trailer-supply knob to make sure they hold the trailer.

Question 9 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 10 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 11 of 20

When releasing brakes after a hard stop, you should:

Continue holding the brakes
Release smoothly to allow air to refill the tanks
Pump the brakes
Apply the parking brake immediately

Correct. Release the brakes smoothly. Air tanks refill while you drive; allow the compressor to recharge the system before the next braking event.

Question 12 of 20

You are driving a fully loaded combination vehicle on a steep downgrade. Your safe speed is 30 mph and you have already shifted to a lower gear. The proper braking technique is to:

Hold the brakes lightly the whole way
Apply firmly to slow to 25 mph (5 mph below safe speed), release, and let speed build back to 30, then repeat
Stay at 30 mph
Apply only the trailer brake

Correct. The "snub-and-release" method: brake firmly to drop 5 mph below safe speed, release until safe speed returns, repeat. Holding the brakes overheats them.

Question 13 of 20

Brake fade is the result of:

Cold brakes
Worn linings only
Heat from continuous use, which reduces braking effectiveness
Air leaks

Correct. Brake fade comes from heat. Manage downgrade speed by gear selection plus intermittent firm brake use, not continuous light pressure.

Question 14 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 15 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 16 of 20

You are driving a tractor-trailer with the air-pressure gauge dropping rapidly. You should:

Continue to your destination
Pull over safely as soon as possible
Apply the brakes hard
Speed up to reach a safe place

Correct. A rapid drop in air pressure indicates a serious leak. Pull over before the pressure drops to the point where the spring brakes apply and the vehicle stops on the road.

Question 17 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 18 of 20

During a brake check, the brake pedal should not move more than ___ when you push it down hard with the engine off and air pressure built up:

One inch
A few inches
To the floor
Half its travel

Correct. After full charge, holding the foot valve down should produce a firm pedal that moves only a few inches. Pedal sinking to the floor indicates a leak.

Question 19 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 20 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.

About the Oklahoma 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 Service Oklahoma Driver License Services 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 Oklahoma CDL practice tests.