SD · L (restriction removed) Endorsement

South Dakota Air Brakes CDL Practice Test

Below are 25 exam-style questions for the South Dakota Air Brakes CDL knowledge test, modeled on the FMCSA-aligned content used by the South Dakota Driver Licensing Program. 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

Air pressure should normally build from 50 to 90 psi within:

A It does not matter
B About 3 minutes in dual systems
C Less than 30 seconds
D 10 minutes

Correct answer: B

Dual air systems should build from 50 to 90 psi within about 3 minutes at idle.

Question 2 of 25

When applying brakes in an emergency without ABS:

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

Correct answer: B

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

Question 3 of 25

A brake check ahead of a downgrade should be done:

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

Correct answer: A

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

Question 4 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 5 of 25

The air compressor governor controls:

A Engine RPM
B Coolant temperature
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 6 of 25

Each axle group on an air-brake-equipped vehicle has its own:

A Set of brake chambers
B Drive shaft
C Engine
D Air compressor

Correct answer: A

Each axle group has its own brake chambers fed by the air system; one compressor supplies all of them.

Question 7 of 25

You should test the parking brake by:

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

Correct answer: C

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

Question 8 of 25

Modulating valves on the rear axle help prevent:

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

Correct answer: C

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

Question 9 of 25

You should test the trailer service brakes by:

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

Correct answer: B

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

Question 10 of 25

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

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

Correct answer: D

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

Question 11 of 25

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

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

Correct answer: C

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

Question 12 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 1 psi per minute
C 2 psi per minute
D 3 psi per minute

Correct answer: C

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

Question 13 of 25

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

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

Correct answer: C

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

Question 14 of 25

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

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

Correct answer: A

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

Question 15 of 25

After parking a tractor-trailer:

A Set only the trailer parking brake
B Leave both released
C Set both tractor and trailer parking brakes
D Set only the tractor 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 16 of 25

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

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

Correct answer: A

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

Question 17 of 25

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

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

Correct answer: B

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

Question 18 of 25

When applying brakes in an emergency with ABS:

A Stab the brakes
B Press as hard as possible and hold
C Release the brakes immediately
D Pump rapidly

Correct answer: B

With ABS, full pressure works because the system pulses for you, allowing maximum braking while preserving steering.

Question 19 of 25

You should test the service brakes by:

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

Correct answer: A

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

Question 20 of 25

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

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

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 21 of 25

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

A 60 psi
B 85 psi
C 125 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 22 of 25

Cross-leakage between primary and secondary brake systems is:

A Caused by overuse
B Required by federal law
C A serious defect that can prevent backup braking
D A normal feature

Correct answer: C

Each circuit must remain independent so a failure in one still leaves the other working.

Question 23 of 25

On a vehicle with dual air brakes, the warning device must come on before pressure in either system drops below:

A Never; only the gauge needs to read it
B 20 psi
C 40 psi
D 60 psi

Correct answer: D

The low-air warning must activate before pressure drops below 60 psi in either circuit.

Question 24 of 25

When approaching a long downgrade, you should:

A Coast in neutral
B Skip the brake test
C Test the brakes by lightly applying them at the top
D Test the brakes at the bottom

Correct answer: C

A light brake application at the top tests for pulling or weakness before the descent puts heat into the system.

Question 25 of 25

Spring brakes do NOT replace:

A The horn
B The headlights
C The service brakes for normal stops
D The tail lights

Correct answer: C

Spring brakes are for parking and emergency only — never for normal service braking.

Study tips for the South Dakota Air Brakes exam

The Air Brakes portion of the South Dakota CDL exam is graded out of the bank of questions the South Dakota Driver Licensing Program draws from each year. While the exact bank is not published, every question is sourced from the Air Brakes chapter of the South Dakota 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 South Dakota 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 South Dakota Driver Licensing Program 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 South Dakota 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 South Dakota 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 South Dakota Driver Licensing Program office.

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

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