Kentucky Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the Kentucky 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 Kentucky Department of Vehicle Regulation 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 Kentucky.

Question 1 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 2 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 3 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 4 of 20

If a trailer breaks away, the emergency brakes on the trailer will be applied because:

The driver pulled the parking brake
The air supply to the trailer is interrupted, releasing the spring brakes on the trailer
The trailer wiring shuts down
The trailer ABS engages

Correct. When the air line breaks (or is disconnected), trailer air pressure drops and the trailer spring brakes apply, stopping the trailer.

Question 5 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 6 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 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

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 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

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 11 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 12 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 13 of 20

The trailer air-supply control:

Is a yellow diamond knob
Is a red eight-sided (octagonal) knob
Is a green knob
Is the same knob as the parking brake

Correct. The trailer air-supply (tractor protection) is a red eight-sided knob. Push in to supply air to the trailer; pull out to shut off air.

Question 14 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 15 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 16 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 17 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 18 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 19 of 20

Air-brake disc brakes use:

Air to apply force to a clamp on a rotating disc, instead of a drum
Hydraulic fluid
A separate fuel system
Spring force only

Correct. Disc brakes use a brake chamber and slack adjuster like S-cam drum brakes, but the force closes a caliper on a rotor instead of forcing shoes against a drum.

Question 20 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.

About the Kentucky 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 Kentucky Department of Vehicle Regulation 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 Kentucky CDL practice tests.