Nebraska Air Brakes CDL Practice Test

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

Question 1 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 2 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 3 of 20

Pre-trip air-brake check (Step 1) is to:

Test the parking brake
Test the air leakage rate
Time the air-pressure build-up
Check the slack adjusters

Correct. In the seven-step air-brake check the first step is testing the parking brake — release service brakes, set parking brake, gently try to move the vehicle in low gear.

Question 4 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 5 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 6 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 7 of 20

In a fully charged dual air-brake system at idle, you should test the low-pressure warning by:

Letting the air bleed down with the engine off and brakes off, fanning the brakes if needed; warning should activate at or before 60 psi
Holding the foot valve down for one minute
Removing the alternator belt
Disconnecting the compressor

Correct. Engine off, fan the brake pedal to bleed pressure. The low-pressure warning device should activate before pressure drops below 60 psi.

Question 8 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 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

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 11 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 12 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 13 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 14 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 15 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 16 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 17 of 20

The air compressor is driven by:

The transmission
The engine, usually by gears or belt
An electric motor
A separate engine

Correct. The air compressor is engine-driven, typically through gears at the front of the engine or by a v-belt.

Question 18 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 19 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 20 of 20

Modulating valves on the trailer:

Don't exist
Apply increasing brake pressure as you push the foot valve harder
Are emergency-only
Open the parking brake

Correct. The trailer service brakes are modulated — pressing harder on the foot valve produces more brake-chamber pressure at the trailer.

About the Nebraska 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 Nebraska 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 Nebraska CDL practice tests.