Free South Dakota DMV CDL — Air Brakes Practice Test 2026
The South Dakota CDL — Air Brakes knowledge test has ~25 questions, and you need ~20 correct (80%) to pass. Practice free with questions written from the official South Dakota driver handbook — every one has a clear explanation, and you can retake as often as you like with no sign-up.
Content reviewed against official driver handbooks · August 12, 2026
What's on the South Dakota written test
The South Dakota CDL — Air Brakes knowledge test has ~25 questions; you need ~20 correct (80%) to pass.
Based on the official South Dakota exam format — always confirm current details with your state DMV.
Practice by topic
Focus on one area at a time — each topic has its own set of South Dakota practice questions.
Following distance, scanning, blind spots, using your lights, and the habits that prevent crashes and keep you in control.
What to do when things go wrong — brake failure, tire blowouts, skids, and stalling on railroad tracks.
The point system, license suspension, provisional-license rules, insurance requirements, and other licensing basics.
How to pass the first time
- Read the official South Dakota driver handbook at least once before test day.
- Practice by topic to find your weak spots, then fix them.
- Take full-length practice tests until you pass comfortably every time.
- Read every explanation — understanding beats memorizing.
South Dakota CDL — Air Brakes test — frequently asked questions
How many questions are on the South Dakota CDL — Air Brakes test?
The South Dakota CDL — Air Brakes knowledge test has ~25 questions, and you need ~20 correct (80%) to pass. Our practice tests mirror that format.
How many questions can I miss on the South Dakota CDL — Air Brakes test?
You can miss up to ~5 of the ~25 questions and still pass — you need ~20 correct, which is 80%.
Can I retake the practice test?
Yes — practice as often as you like, completely free. The questions and answer choices are shuffled each time.
Can I take the South Dakota practice test in Spanish?
Yes. Every question here is available in English and Spanish — switch languages anytime. Many states also offer the real knowledge test in languages other than English; confirm what South Dakota offers with your DMV.
Can I practice on my phone?
Yes. The practice tests work in any mobile or desktop browser — no app or sign-up needed — so you can study anywhere.
Are these the real DMV test questions?
No. Our questions are original, written from the current official South Dakota driver handbook to match the real test's topics and style. We are not affiliated with any DMV or government agency.
What's the best way to study?
Read the South Dakota driver handbook, practice by topic to target weak areas, and finish with full-length tests until you consistently pass.
Other South Dakota practice tests
Take the CDL — Air Brakes test in another state
Every South Dakota question is written from the official South Dakota driver handbook and checked against its current edition. DMV Test Free is a free, independent study resource — not affiliated with any DMV or government agency. About DMV Test Free
South Dakota CDL — Air Brakes questions with answers
20 sample questions from the South Dakota CDL — Air Brakes question bank — each with the correct answer, a short explanation, and the section of the official handbook it comes from.
Show all 20 questions with answers
1. During the pressure buildup check, your air pressure rises more slowly than the manual's standard. What risk does the manual point to?
Correct answer: Pressure may fall too low while you drive and force an emergency stop
A compressor that cannot rebuild pressure quickly may not keep up with normal brake use, so pressure could sink too far on the road and leave you needing an emergency stop. The manual says not to drive until the problem is fixed.
Source: South Dakota Commercial Driver License Manual — Section 5.3.3, Step 7 Final Air Brake Check
2. Which of these parts plays NO role in applying an s-cam drum brake?
Correct answer: The power screw
In an s-cam brake, air entering the brake chamber drives the rod outward, the rod moves the slack adjuster, and the slack adjuster twists the camshaft that turns the s-cam. The power screw belongs to air-operated disc brakes, where it takes the place of the s-cam.
Source: South Dakota Commercial Driver License Manual — Section 5.1.8, Foundation Brakes
3. You pass South Dakota's Air Brakes Test but put the rest of your licensing on hold. How long do a passed knowledge test and its fee stay valid?
Correct answer: 6 months
The manual says every passed knowledge test, along with the fee paid, remains valid for 6 months. Plan the skills test and the remaining steps inside that window.
Source: South Dakota Commercial Driver License Manual — General Information, Knowledge Tests
4. At 55 mph on dry pavement, about how much distance does air brake lag add to your stop?
Correct answer: About 32 feet
The manual puts brake lag distance at roughly 32 feet at that speed. It is one of four parts of total stopping distance, which for an average driver with good traction and brakes comes to more than 450 feet at 55 mph.
Source: South Dakota Commercial Driver License Manual — Section 5.4.4, Stopping Distance
5. You have just come down a steep grade and your brakes are very hot. What does the manual say about setting the parking brakes right away?
Correct answer: Do not: the heat can damage them, so let the brakes cool first
Parking brakes applied on very hot brakes can be damaged by the heat. The manual has you wait for them to cool, holding the vehicle meanwhile with wheel chocks on level ground.
Source: South Dakota Commercial Driver License Manual — Section 5.4.8, Parking Brakes
6. In a dual air brake system, one system's pressure falls very low while the other holds. What does the manual say that means for your braking?
Correct answer: The front brakes or the rear brakes will be working at less than full strength, so stopping takes longer
Each half of a dual system serves different axles, so when one half is very low the brakes it feeds, front or rear, are not fully working and the stop gets longer. The manual's instruction is to stop safely and get the air brake system repaired.
Source: South Dakota Commercial Driver License Manual — Section 5.2, Dual Air Brake
7. A truck's dash has a supply pressure gauge and an application pressure gauge. Which of the two does the manual say every vehicle with air brakes has?
Correct answer: The supply pressure gauge
Every air-braked vehicle has a supply pressure gauge tied to the air tank, showing how much pressure the tanks hold. The application pressure gauge, which shows the pressure you are sending to the brakes, is not fitted to all vehicles.
Source: South Dakota Commercial Driver License Manual — Section 5.1.9, Supply Pressure Gauges
8. Which two codes on South Dakota's list of commercial driver license restrictions deal with air brakes?
Correct answer: L and Z
On South Dakota's restriction list, L means no air brakes and Z means no full air brakes (the driver tested with air over hydraulic brakes). E is no manual transmission, K is intrastate only, M and N limit passenger vehicles, O is a non-fifth-wheel CMV and V is a medical variance.
Source: South Dakota Commercial Driver License Manual — General Information, SD License Restrictions
9. With air brakes, about how long does the manual say it takes for air to travel through the lines to the brakes after you push the pedal?
Correct answer: One half second or more
Hydraulic brakes act instantly, but air needs a short time, half a second or longer, to reach the brakes. That delay is brake lag, and the distance covered during it is added to the stopping distance.
Source: South Dakota Commercial Driver License Manual — Section 5.4.4, Stopping Distance
10. In the final air brake check you keep fanning the pressure down until the spring brakes apply. On a single vehicle such as a straight truck, which valve should pop out?
Correct answer: The parking brake valve
On a tractor-trailer, the tractor protection valve and the parking brake valve should both close. On other combinations and on single vehicles it is the parking brake valve that pops out, at the manufacturer's specified pressure somewhere in the 20 to 45 psi range, and the spring brakes then come on.
Source: South Dakota Commercial Driver License Manual — Section 5.3.3, Step 7 Final Air Brake Check
11. Air pressure has fallen far enough that the spring brakes come on in a heavily loaded truck. Why does the manual say the stop will take a long distance?
Correct answer: Spring brakes do not act on every axle
Spring brakes are not fitted on all axles, so a heavy vehicle has less braking than usual once they apply. That is why the manual calls it much safer to stop while the tanks still hold enough air for the foot brake.
Source: South Dakota Commercial Driver License Manual — Section 5.4.7, Low Air Pressure
12. Automatic front wheel limiting valves, found on many vehicles, cut back the air sent to the front brakes. When do these valves stop limiting it?
Correct answer: When the brakes are applied very hard, at 55 psi or more of application pressure
Automatic front wheel limiting valves hold back air to the front brakes during ordinary braking. A very hard application, 55 psi or more on the application side, is the exception, and then the front brakes get full air.
Source: South Dakota Commercial Driver License Manual — Section 5.1.13, Front Brake Limiting Valve
13. How many questions are on South Dakota's Air Brakes knowledge test?
Correct answer: 25
South Dakota's list of CDL knowledge tests gives the Air Brakes Test 25 multiple-choice questions. The General Knowledge Test has 50, the Hazardous Materials Test has 30, and most of the other endorsement tests have 20.
Source: South Dakota Commercial Driver License Manual — General Information, Knowledge Tests
14. You are in the proper low gear on a long downgrade and have judged 30 mph to be your "safe" speed. Using the manual's braking technique, to about what speed do you slow before releasing the brakes?
Correct answer: About 25 mph
The technique is to brake once your speed reaches the safe speed, slow to about five mph under it, then release. With a 30 mph safe speed that means easing down to roughly 25 mph. The manual's own example uses 40 mph and 35 mph.
Source: South Dakota Commercial Driver License Manual — Section 5.4.6, Proper Braking Technique
15. How does the manual's final air brake check have you test the parking brake?
Correct answer: Try to ease forward in a low gear with the vehicle stopped and the brake set, and confirm it holds
The test is simple: stop, apply the parking brake, then use a low gear to tug lightly against it. If the vehicle stays put, the brake is holding.
Source: South Dakota Commercial Driver License Manual — Section 5.3.3, Step 7 Final Air Brake Check
16. At highway speed your low air warning light and buzzer come on. The gauge is still above the range where spring brakes apply, and the next exit is several miles ahead. What does the manual tell you to do?
Correct answer: Stop safely right away, while the brakes are still yours to control
The manual says not to wait for the spring brakes to apply by themselves. As soon as the light and buzzer appear, you make a safe stop at once, because the brakes still respond to you at that point and may not later.
Source: South Dakota Commercial Driver License Manual — Section 5.1.14, Spring Brakes
17. During the emergency brake test on the vehicle inspection test, your parking brake valve stays in until the gauge reads about 15 psi. No manufacturer specification says otherwise. How does that compare with Section 11's standard?
Correct answer: It is too low: the valve should close no lower than 20 psi
Section 11 says the parking (or emergency) brake valve should pop out between 20 and 45 psi, meaning not below 20 and not above 45, unless the manufacturer specifies a different level. A valve that waits until 15 psi is outside that window.
Source: South Dakota Commercial Driver License Manual — Section 11.1.2, Air Brake or Hydraulic Brake Check
18. What score does South Dakota require to pass the Air Brakes Test, like every other CDL knowledge test?
Correct answer: 80% or better
The manual sets one passing mark for each CDL knowledge test: at least 80%. That applies to the 25-question Air Brakes Test just as it does to the General Knowledge Test.
Source: South Dakota Commercial Driver License Manual — General Information, Knowledge Tests
19. During South Dakota's vehicle inspection test you leave the cab to continue the inspection outside. What does Section 11 require as you exit the vehicle?
Correct answer: Set the parking brake or brakes and put the vehicle in neutral
Section 11 requires two things each time you get out during the test: the parking brake or brakes set, and the transmission in neutral. It also gives forgetting to set the parking brake as an example of an unsafe act, which may bring an automatic failure and end the test.
Source: South Dakota Commercial Driver License Manual — Section 11, Vehicle Inspection Test
20. On a wet road in a truck without ABS, you brake hard and turn the steering wheel to miss a stalled car, but the truck keeps sliding straight ahead. What does the manual's explanation point to?
Correct answer: The steering wheels have locked up
When the steering wheels lock, they slide instead of rolling, and the manual says steering control is lost. Lockup at the other wheels shows up differently: the vehicle can skid, jackknife or spin. ABS is there to help you avoid wheel lockup.
Source: South Dakota Commercial Driver License Manual — Section 5.4.2, Braking with Antilock Brakes