Oregon Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the Oregon 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 Oregon Driver and Motor Vehicle 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 Oregon.

Question 1 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 2 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 3 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 4 of 20

You should never leave the vehicle unattended without:

Locking the doors
Applying the parking brakes and chocking the wheels if necessary
Turning off the radio
Removing the key

Correct. Set the parking brake. On grades or where unintended movement could occur, chock the wheels in addition.

Question 5 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 6 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 7 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 8 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 9 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 10 of 20

If you must drive a vehicle with manual front-wheel-brake limiting valve, you should keep it in the "normal" position:

Always
Only on slick roads
In the slippery position only
Only when empty

Correct. Keep the front-wheel limiting valve in the "normal" position. Modern trucks rarely have this valve, but if equipped, leaving it in "slippery" reduces front braking and lengthens stopping distance.

Question 11 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 12 of 20

A yellow ABS malfunction lamp on the trailer (typically left side) means:

The trailer brakes have failed
The trailer ABS is malfunctioning
The trailer is overweight
Tire pressure is low

Correct. A trailer-mounted yellow ABS lamp indicates the trailer's ABS is malfunctioning. Service brakes still work; have the system repaired.

Question 13 of 20

You hear a steady "ssss" sound while parked with the engine off. This is most likely:

Normal venting
An air leak that needs immediate attention
The compressor running
Trailer brakes releasing

Correct. A steady hiss with the engine off indicates an air leak — find and repair before driving.

Question 14 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 15 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 16 of 20

Air loss from the brake system on a moving combination vehicle, with brakes applied, should not exceed:

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

Correct. Combination vehicle, brakes applied, engine off: no more than 4 psi/min air loss. Engine off, brakes released: no more than 3 psi/min.

Question 17 of 20

You should never use the parking brake when the brakes are very hot because:

It will not hold
It can damage the brakes or cause warpage
It will release on its own
It will use too much air

Correct. Hot brakes cooled with parking brake applied can warp the drum or rotor, or cause the spring brake to fail to release.

Question 18 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 19 of 20

Total stopping distance with air brakes equals:

Perception distance + reaction distance + brake-lag distance + effective braking distance
Reaction distance only
Brake-lag distance only
Perception distance + braking distance

Correct. Add brake-lag distance to the standard perception + reaction + braking distance for any vehicle with air brakes.

Question 20 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.

About the Oregon 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 Oregon Driver and Motor Vehicle 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 Oregon CDL practice tests.