OH · L (restriction removed) Endorsement

Ohio Air Brakes CDL Practice Test

Below are 25 exam-style questions for the Ohio Air Brakes CDL knowledge test, modeled on the FMCSA-aligned content used by the Ohio Bureau of Motor Vehicles. 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

The air-brake hand valve (trolley valve) operates:

A Both tractor and trailer brakes
B The parking brake
C The trailer service brakes only
D The tractor service brakes only

Correct answer: C

The hand valve applies only the trailer service brakes. It is not a parking brake or a substitute for the foot brake.

Question 2 of 25

After parking a tractor-trailer:

A Set only the tractor parking brake
B Leave both released
C Set both tractor and trailer parking brakes
D Set only the trailer parking brake

Correct answer: C

Federal practice is to set both parking brakes when fully parked. The exception is during coupling/uncoupling, where the trailer brakes are set.

Question 3 of 25

Stopping a vehicle with an air-brake failure can sometimes be done by:

A Honking
B Using the trailer hand valve, then engine braking, then a runaway ramp if needed
C Putting the transmission in reverse
D Coasting in neutral

Correct answer: B

In an air-brake failure, the trailer hand valve can still apply the trailer service brakes, then engine braking and a runaway ramp.

Question 4 of 25

Slack adjusters should be checked because:

A They are decorative
B Out-of-adjustment slack adjusters can result in brakes that do not work properly
C They control trailer height
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 5 of 25

The air compressor is driven by:

A The electrical system
B The transmission
C The engine, by belts or directly geared
D The exhaust system

Correct answer: C

Compressors are typically engine-driven, either by a belt or direct gearing, so air pressure builds whenever the engine runs.

Question 6 of 25

A brake check ahead of a downgrade should be done:

A Before reaching the top, while still on level ground
B In the middle of the descent
C Once a year
D At the bottom only

Correct answer: A

A brief brake test on level ground at the top reveals problems while you can still stop safely.

Question 7 of 25

Brake fade is most likely caused by:

A Cold weather
B Too much air pressure
C Worn-out hoses
D Heat from continuous brake use on long downgrades

Correct answer: D

Heat reduces friction and brake effectiveness. Use a low gear so the engine, not the service brakes, holds the load.

Question 8 of 25

When doing the air-leak rate test, the engine should be:

A Running at high RPM
B Started and stopped repeatedly
C In gear
D Off, with brakes released for the first part

Correct answer: D

Engine off, brakes released to test static leak rate; then brakes applied and held for the second part.

Question 9 of 25

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

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

Correct answer: C

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

Question 10 of 25

If air pressure drops in the emergency line:

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

Correct answer: D

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

Question 11 of 25

When testing low-air warning, you should:

A Look at the dashboard light
B Pump the brake to fan down the pressure and verify the warning activates before pressure drops below 60 psi
C Disconnect the trailer
D Drain the wet tank only

Correct answer: B

Fan the brakes (engine off) until the warning device activates and confirm it is above 60 psi.

Question 12 of 25

The total stopping distance for an air-brake equipped vehicle is:

A Reaction + braking + brake-lag distance
B Perception + braking distance
C Perception + reaction + brake-lag + braking distance
D Reaction distance + braking distance

Correct answer: C

Air brakes add a brake-lag distance — the time from foot-pressure to actual brake application — that hydraulic systems do not have.

Question 13 of 25

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

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

Correct answer: A

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 14 of 25

A foot-valve pressure gauge reading lower than expected during a brake application could indicate:

A A worn seat belt
B Normal operation
C A leak or restriction
D A new compressor

Correct answer: C

Low pressure during application means the system isn't delivering full braking force — investigate.

Question 15 of 25

Modulating valves on the rear axle help prevent:

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

Correct answer: B

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

Question 16 of 25

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

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

Correct answer: B

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 17 of 25

You should test the service brakes by:

A Listening to the air gauge
B At about 5 mph, applying the brake firmly and feeling for pulling, sticking, or unusual feel
C Driving at 30 mph and slamming on the brakes
D Pumping the brakes

Correct answer: B

A low-speed brake test catches sticking, pulling, or weak brakes before you build up speed.

Question 18 of 25

The proper braking technique on a long downgrade is:

A Apply the parking brake
B Light, intermittent braking with engine braking and a low gear
C Coast in neutral
D Heavy continuous braking

Correct answer: B

Use the proper low gear so engine braking does most of the work; brief, moderate brake applications keep the speed in check.

Question 19 of 25

Air-brake hoses should:

A Be coiled tightly under the truck
B Have at least 5 splices each
C Show no signs of damage, leaks, or excessive wear
D Be wrapped in tape

Correct answer: C

Damaged or chafed hoses are a common air-brake defect and a frequent out-of-service citation.

Question 20 of 25

You should test the parking brake by:

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

Correct answer: D

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

Question 21 of 25

You should test the trailer service brakes by:

A Listening for a hiss
B Releasing the parking brakes, moving the vehicle slowly forward, and applying the trailer hand valve
C Pumping them while parked
D Looking at the gauge

Correct answer: B

A low-speed pull-and-stop with the trailer hand valve confirms the trailer brakes apply on their own.

Question 22 of 25

In a single-vehicle air-brake check with the engine off and brakes released, the maximum allowable air loss is:

A 5 psi per minute
B 3 psi per minute
C 1 psi per minute
D 2 psi per minute

Correct answer: D

Single vehicle, brakes released: not more than 2 psi per minute. Combination vehicle: 3 psi per minute.

Question 23 of 25

When applying brakes in an emergency without ABS:

A Use stab braking — apply hard, release when wheels lock, re-apply
B Press as hard as possible and hold
C Use only the parking brake
D Pump rapidly and lightly

Correct answer: A

Stab braking keeps the truck straight in an emergency without ABS. Hard continuous pressure can lock the wheels and cause a jackknife.

Question 24 of 25

When applying the foot brake hard:

A Air pressure may drop slightly
B The compressor cuts out
C Air pressure should remain unchanged
D Air pressure increases

Correct answer: A

A small drop is expected during heavy application. The compressor recharges the system back up to cut-out pressure.

Question 25 of 25

After releasing the parking brake on a tractor, you should:

A Pump the service brake five times
B Drive immediately
C Drain the wet tank
D Wait for full system pressure before moving

Correct answer: D

Make sure the system reaches at least the cut-out pressure (typically around 120 psi) before driving.

Study tips for the Ohio Air Brakes exam

The Air Brakes portion of the Ohio CDL exam is graded out of the bank of questions the Ohio Bureau of Motor Vehicles draws from each year. While the exact bank is not published, every question is sourced from the Air Brakes chapter of the Ohio 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 Ohio 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 Ohio Bureau of Motor Vehicles 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 Ohio 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 Ohio 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 Ohio Bureau of Motor Vehicles office.

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

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