KY · L (restriction removed) Endorsement

Kentucky Air Brakes CDL Practice Test

Below are 25 exam-style questions for the Kentucky Air Brakes CDL knowledge test, modeled on the FMCSA-aligned content used by the Kentucky Transportation Cabinet. Try to answer each question on your own before reading the answer key directly under it. The questions and answer choices are shuffled deterministically per state and endorsement, so the order will stay the same on repeat visits — that lets you genuinely measure your improvement.

Heads up: this is a study tool, not a graded exam. Cover the answer with your hand or a sheet of paper for an honest practice run, then re-read the explanations for any questions you missed. Aim for 22 out of 25 or better, three times in a row, before scheduling the real exam.

Question 1 of 25

A correctly adjusted brake will have:

A Loose drum bolts
B Pushrod travel within the legal limit for that brake type
C Visible rust
D No slack adjuster

Correct answer: B

Pushrod travel must be within the manufacturer's and federal limits — exact value depends on chamber size.

Question 2 of 25

Air dryers are used in air-brake systems to:

A Increase pressure
B Replace governors
C Cool the air before it enters the brake chambers
D Remove moisture and contaminants from the compressed air

Correct answer: D

Dryers reduce moisture in the air system, helping prevent corrosion, ice in winter, and contamination.

Question 3 of 25

A common reason for compressor failure to keep up is:

A A clogged air filter or worn compressor
B A broken governor
C All of the above
D A leak in the brake system

Correct answer: C

Any of these causes can prevent the compressor from maintaining cut-out pressure.

Question 4 of 25

Which is the proper procedure when the low-air warning comes on while driving?

A Bring the vehicle to a safe stop as soon as possible and find the cause
B Pump the brakes to keep pressure
C Continue to the next exit
D Increase engine RPM

Correct answer: A

Continued driving below safe air pressure risks losing service brakes entirely. Stop safely and diagnose.

Question 5 of 25

Why should you NOT use the trailer hand valve to hold a parked combination vehicle?

A It will activate the spring brakes
B Air leaks could cause the brakes to release and let the vehicle roll
C It is illegal
D It only works while the engine is running

Correct answer: B

The hand valve relies on continuous air pressure. A slow leak releases the brakes and the vehicle rolls.

Question 6 of 25

When the parking brake is set, the dashboard valve will normally:

A Stay popped out and yellow
B Stay pushed in
C Have no indication
D Flash red

Correct answer: A

Yellow diamond-shaped valves stay out (popped) when parking brakes are applied; pushing them in releases the brakes.

Question 7 of 25

A driver should never:

A Use the trailer hand valve as a parking brake
B Test the parking brake
C Make pre-trip inspections
D Drain the wet tank

Correct answer: A

The trailer hand valve is not a parking device — see the trailer-hand-valve question. Always set the trailer parking brakes.

Question 8 of 25

If air pressure drops in the emergency line:

A Nothing happens
B Trailer service brakes apply
C Trailer spring brakes apply automatically
D Tractor brakes apply

Correct answer: C

Loss of supply-line pressure is the failsafe that triggers the trailer's spring brakes.

Question 9 of 25

Spring brakes are held off by:

A Engine vacuum
B Electrical current
C Hydraulic pressure
D Air pressure

Correct answer: D

Compressed air holds the springs back. When air pressure drops, the springs apply the brakes mechanically.

Question 10 of 25

A dual air-brake system means:

A Twice the air pressure
B Two compressors
C Two governors
D Two separate air-brake systems on one set of brake controls

Correct answer: D

Modern trucks have two separate air systems (often labeled primary and secondary) with one set of brake controls so a failure in one circuit still leaves working brakes.

Question 11 of 25

The air compressor governor controls:

A Coolant temperature
B Engine RPM
C When the air compressor will pump air into the storage tanks
D Brake pad wear

Correct answer: C

The governor cuts the compressor in (start pumping) at low pressure and cuts it out (stop pumping) at high pressure to maintain a working range.

Question 12 of 25

Brake-lag distance for a CMV traveling 55 mph is approximately:

A 0 feet
B 300 feet
C 32 feet
D 142 feet

Correct answer: C

About 32 feet at 55 mph for the brake lag alone — added to reaction and braking distances.

Question 13 of 25

A common cause of an air-brake skid is:

A Locking the wheels by braking too hard
B Driving too fast for conditions
C Neither of the above
D Both of the above

Correct answer: D

Skids result when the wheel locks and the tire loses traction. Speed and over-application are both common contributors.

Question 14 of 25

A leaking air system on a parked truck is dangerous because:

A The engine will not start
B It can wake the driver
C When pressure drops far enough, spring brakes apply suddenly and the vehicle becomes unmovable
D The fuel will leak

Correct answer: C

A bigger problem is en route: if a slow leak goes unnoticed and pressure drops below the spring-brake set point, the brakes apply on the road.

Question 15 of 25

In a combination vehicle, with the brakes applied and the engine off, the maximum allowable air loss is:

A 3 psi per minute
B 5 psi per minute
C 2 psi per minute
D 4 psi per minute

Correct answer: D

Combination vehicle, brakes applied: 4 psi per minute. Single vehicle, brakes applied: 3 psi per minute.

Question 16 of 25

Anti-lock braking systems (ABS) on air-brake vehicles:

A Replace foundation brakes
B Help prevent wheel lockup but do not necessarily shorten stopping distance
C Are required only on hazmat trailers
D Are optional and rarely installed

Correct answer: B

ABS helps the driver maintain steering control by preventing wheel lockup. It does not necessarily reduce stopping distance.

Question 17 of 25

In a dual system, normal cut-in pressure is approximately:

A 40 psi
B 60 psi
C 125 psi
D 85 psi

Correct answer: B

Cut-in is usually around 100 psi but cut-out is around 125 psi. Cut-in below 60 psi indicates a problem in many systems.

Question 18 of 25

When you press the brake pedal, brake pressure is:

A Released from the storage tanks
B Created by the pedal mechanically
C Built up from zero by the pedal
D Received from a separate accumulator

Correct answer: A

Pressing the pedal releases stored air from the tanks into the brake chambers; the pedal does not generate pressure itself.

Question 19 of 25

Slack adjusters should be checked because:

A They control trailer height
B Out-of-adjustment slack adjusters can result in brakes that do not work properly
C They are decorative
D They affect engine performance

Correct answer: B

Slack adjusters convert pushrod motion into braking force. Out-of-adjustment slack adjusters reduce braking ability and are a common out-of-service item.

Question 20 of 25

Glad-hand seals should be:

A Free of dirt and damage and properly seated
B Removed for inspection
C Coated with oil
D Loose for easy connection

Correct answer: A

Damaged or dirty seals cause leaks. Inspect and clean them as part of the trailer hookup.

Question 21 of 25

Modulating valves on the rear axle help prevent:

A Wheel lockup during emergency braking
B Steering wander
C Engine overheating
D Tire blowouts

Correct answer: A

Modulating valves and ABS sensors release pressure briefly to keep wheels from locking.

Question 22 of 25

You should test the parking brake by:

A Doing nothing — the dashboard light is enough
B Pressing the service brake while parked
C Setting the parking brake, releasing the service brake, and gently trying to move the vehicle
D Driving over a curb

Correct answer: C

Apply throttle gently in low gear; if the truck moves, the parking brake is not holding.

Question 23 of 25

During the seven-step air-brake check, the final step is to:

A Check tire pressures
B Drain the wet tank
C Pull forward and apply the foot brake to test for stopping
D Adjust the slack adjusters

Correct answer: C

After all stationary tests, perform a moving brake check at low speed to verify the service brakes stop the vehicle.

Question 24 of 25

If the ABS warning light comes on while driving, you should:

A Disconnect the trailer
B Pump the brakes hard
C Continue driving — you still have normal brakes — and have the system checked at the next opportunity
D Immediately stop on the side of the road

Correct answer: C

A failed ABS system reverts to normal braking. Get it repaired but you can complete the trip.

Question 25 of 25

You should not over-tighten a glad-hand because:

A It is bad luck
B It violates federal weight law
C It will reduce engine power
D It can damage the rubber seals

Correct answer: D

Over-tightening crushes the seals. The connection should be firm but not forced.

Study tips for the Kentucky Air Brakes exam

The Air Brakes portion of the Kentucky CDL exam is graded out of the bank of questions the Kentucky Transportation Cabinet draws from each year. While the exact bank is not published, every question is sourced from the Air Brakes chapter of the Kentucky CDL handbook, which itself is derived from the FMCSA Model Commercial Driver's License Manual. That means studying our practice tests, reading the corresponding handbook chapter, and re-reading the parts you got wrong is genuinely the most efficient route to a first-time pass.

Most successful applicants follow a simple cycle: take the practice test cold, write down every question you missed, open the matching chapter of the official Kentucky handbook, re-read the section that contains the right answer, then re-take the practice test 24 to 48 hours later. The 24-hour delay matters — sleep is when your brain commits new information to long-term memory, and CDL knowledge questions reward that kind of consolidated learning rather than cramming.

Pay particular attention to questions that include qualifier words like always, never, only, primary, or most. CDL test writers love to flip the right answer with a single qualifier. When two answer choices look almost identical, pay attention to the verb (is it must, should, or may?) and to any numbers (14 days, 100 air miles, 8 hours, 70/8 split). On endorsement tests in particular, watch for trick framing where a true statement about a different endorsement is offered as the "correct" answer to a question that is actually about Air Brakes.

Test-day logistics matter too. Bring photo ID, your Social Security card or birth certificate, your medical examiner's certificate (DOT card), and proof of state residency if you haven't already submitted those documents. The Kentucky Transportation Cabinet will not let you sit for the knowledge exam without your documentation, and most offices charge an additional fee for re-attempts. Arrive early — the wait at most CDL testing offices runs 30 to 60 minutes — and silence your phone before the exam begins.

Finally, keep your General Knowledge fundamentals sharp even when you're focused on the Air Brakes exam. Many states administer multiple knowledge tests in a single sitting, and questions on weight definitions (GVWR, GCWR, GAWR), stopping distance, and the pre-trip inspection routine show up across endorsements. If you're unsure on the basics, sit a fresh Kentucky General Knowledge practice test before scheduling the real exam.

Next steps

Missed more than four questions? Re-read the Air Brakes study guide and the matching chapter in the official Kentucky CDL handbook. Then come back and re-take the test. Once you can score 22 of 25 or higher on three runs in a row, you're in good shape to schedule the real exam at your local Kentucky Transportation Cabinet office.

Already comfortable with this endorsement? Drill another: KY General Knowledge · KY Combination Vehicles · KY Hazardous Materials · KY Passenger · KY School Bus · KY Tank Vehicle · KY Doubles / Triples

New to the CDL process in Kentucky? Read How to apply for a CDL in Kentucky for the document checklist and step-by-step timeline.