Free Idaho DMV CDL — Air Brakes Practice Test 2026
The Idaho CDL — Air Brakes knowledge test has ~25 questions, and you need ~20 correct (80%) to pass. Practice free with questions written from the official Idaho 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 Idaho written test
The Idaho CDL — Air Brakes knowledge test has ~25 questions; you need ~20 correct (80%) to pass.
Based on the official Idaho 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 Idaho practice questions.
How to pass the first time
- Read the official Idaho 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.
Idaho CDL — Air Brakes test — frequently asked questions
How many questions are on the Idaho CDL — Air Brakes test?
The Idaho 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 Idaho 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 Idaho 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 Idaho 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 Idaho 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 Idaho driver handbook, practice by topic to target weak areas, and finish with full-length tests until you consistently pass.
Other Idaho practice tests
Take the CDL — Air Brakes test in another state
Every Idaho question is written from the official Idaho 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
Idaho CDL — Air Brakes questions with answers
20 sample questions from the Idaho 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. Idaho's vehicle inspection test section explains why the air brake check matters. What does it say losing air pressure will do?
Correct answer: Set off the emergency brakes, bringing an abrupt stop the driver does not control
The 'Why Inspect' note says lost air pressure makes the emergency brakes activate, and the stop that follows is abrupt and uncontrolled. That is why the check confirms the safety devices work as pressure falls from normal toward low air.
Source: Idaho Commercial Driver's License Manual — Section 11.1.2, Air Brake or Hydraulic Brake Check
2. Your truck has an automatic front wheel limiting valve. You make a light brake application of about 20 psi while slowing for a traffic light. What is the valve doing?
Correct answer: Reducing the air sent to the front brakes
Automatic front wheel limiting valves trim the air going to the front brakes unless you brake very hard, at 60 psi or more of application pressure. A 20 psi application is well under that, and the driver has no control over these valves.
Source: Idaho Commercial Driver's License Manual — Section 5.1.13, Front Brake Limiting Valve
3. An Idaho CDL applicant fails the air brake written test. According to the note in the manual's Air Brakes section, what is the result?
Correct answer: An air brake restriction is placed on the CDL
The note names two things that put the air brake restriction on an Idaho CDL: not passing the air brake written test, or taking the skills test in a vehicle without air brakes. Either one is enough.
Source: Idaho Commercial Driver's License Manual — Section 5, Air Brakes
4. Trucks, truck tractors and buses must have emergency brakes and parking brakes that are held on mechanically. What does the manual say usually fills both of those needs?
Correct answer: Spring brakes
Spring brakes are what these vehicles usually rely on for both emergency and parking braking. Their springs supply the mechanical force, so the brakes do not depend on air that could leak away over time.
Source: Idaho Commercial Driver's License Manual — Section 5.1.14, Spring Brakes
5. In the manual's stopping distance chart for an air-braked vehicle at 55 mph, which of the four parts is the longest?
Correct answer: Braking distance
At 55 mph the chart shows about 142 feet of perception distance, 61 feet of reaction distance, 32 feet of brake lag and 216 feet of braking distance, 451 feet in all. Braking distance is the largest share.
Source: Idaho Commercial Driver's License Manual — Section 5.4.4, Stopping Distance
6. Two trucks have wig wag low air warnings, one automatic and one the manual reset type. Both have just built pressure back above 55 psi. What is the difference in how the arms get out of the driver's view?
Correct answer: The automatic arm rises by itself; the manual reset arm has to be put back by hand
An automatic wig wag lifts out of view once system pressure goes above 55 psi. With the manual reset kind, the driver has to move the arm out of view, and it stays put only after pressure has passed 55 psi.
Source: Idaho Commercial Driver's License Manual — Section 5.1.11, Low Air Pressure Warning
7. The manual gives a second name for the air storage tanks that the compressor fills. What is it?
Correct answer: Reservoirs
The manual puts the word reservoirs in parentheses right after it first mentions the air storage tanks. Brake chambers sit at the wheels, and glad hands are couplers for trailer air lines.
Source: Idaho Commercial Driver's License Manual — Section 5.1.1, Air Compressor
8. You are getting ready for Idaho's Air Brake knowledge test. How many questions does the manual's chart list for it, and how many must you get right?
Correct answer: 25 questions, with at least 20 correct
The chart near the front of the Idaho manual lists the Air Brake test at 25 questions with a minimum of 20 correct. The other figures belong to different tests on the same chart: 20 and 16 for several endorsements, 30 and 24 for Hazmat, and 50 and 40 for General Knowledge.
Source: Idaho Commercial Driver's License Manual — Knowledge Test Information
9. A diagram in the Idaho manual's Air Brakes section shows the tractor protection valve and the parking brake controls. According to it, what happens when the parking brakes are applied?
Correct answer: The tractor protection valve closes and the spring brakes set together
The diagram's note says applying the parking brakes closes the tractor protection valve and sets the spring brakes in the same motion. It adds that this valve also closes on its own if the air supply drops while you are driving.
Source: Idaho Commercial Driver's License Manual — Section 5.1.15, Parking Brake Controls
10. The manual adds brake lag distance to the stopping distance formula for air-braked vehicles. Between which two other distances does it sit in that formula?
Correct answer: Reaction distance and braking distance
The formula lists perception, reaction, brake lag and braking distance, in that order. Lag falls after you have reacted and pushed the pedal but before the brakes begin to slow the vehicle.
Source: Idaho Commercial Driver's License Manual — Section 5.4.4, Stopping Distance
11. An air line fails on the highway and your normal braking is compromised. Which of the three air brake systems is the one meant to bring the vehicle to a stop when the brake system fails?
Correct answer: The emergency brake system
The emergency brake system borrows parts from the service and parking brake systems so the vehicle can still be stopped after a failure. The service brakes handle normal driving, and the parking brake system works from the parking brake control.
Source: Idaho Commercial Driver's License Manual — Section 5, Air Brakes
12. Which of these is NOT one of the foundation brake types described in the manual's Air Brakes section?
Correct answer: Band brakes
The section describes three kinds of foundation brakes: s-cam drum brakes, wedge brakes and disc brakes. Band brakes are not among them.
Source: Idaho Commercial Driver's License Manual — Section 5.1.8, Foundation Brakes
13. In the Idaho manual's 'What to Study' chart, Section 5 carries an asterisk. What does the footnote tell you?
Correct answer: Read Section 5 when the vehicles you will drive have air brakes
The footnote under the chart points Section 5 at anyone who plans to operate air-brake-equipped vehicles. It is not tied to one license class or one endorsement.
Source: Idaho Commercial Driver's License Manual — Section 1.1, Commercial Driver License Tests
14. The manual describes two different ways the air holding back the spring brakes can be removed. Which pair is it?
Correct answer: The driver releases it with the parking brake control, or a leak drains all the air
From the cab, the driver can use the parking brake control to exhaust the air from the spring brakes, and the springs then apply them. A leak that empties the whole system does the same thing.
Source: Idaho Commercial Driver's License Manual — Section 5.1.14, Spring Brakes
15. Which of these is NOT part of the controlled braking method for an emergency stop?
Correct answer: Holding the brakes on if the wheels lock, until the vehicle stops
In controlled braking you release the brakes if the wheels lock or if you need a larger steering correction, then brake again as soon as you can. Holding locked wheels all the way to a stop is not part of either emergency method the manual describes.
Source: Idaho Commercial Driver's License Manual — Section 5.4.3, Emergency Stops
16. The first air leakage test in the final air brake check begins with a fully charged system. What pressure does the manual give as typical for a full charge?
Correct answer: 125 psi
The manual describes a fully charged system as typically 125 psi. From there you shut the engine off, let off the parking brake and time how fast the pressure falls.
Source: Idaho Commercial Driver's License Manual — Section 5.3.3, Step 7 Final Air Brake Check
17. You clear Idaho's air brake restriction by passing the written air brakes test and then retaking the skills test in an air-braked commercial vehicle. What does the manual say about the restriction from then on?
Correct answer: It is lifted permanently
The note in the Air Brakes section says that after you pass a test in an air-braked vehicle, the restriction is lifted for good. It mentions no trial period and no later retest.
Source: Idaho Commercial Driver's License Manual — Section 5, Air Brakes
18. Idaho's vehicle inspection test section says the parking brake has to work whenever the vehicle is parked. Which situation does it single out?
Correct answer: Parking on a grade or hill
The 'Why Inspect' note for the parking brake check says the brake must function when the vehicle is parked, especially on a grade or hill. A vehicle that rolls could injure other road users or damage property.
Source: Idaho Commercial Driver's License Manual — Section 11.1.1, In-Vehicle/Engine Start
19. Idaho's manual prints a notice for truckers about runaway truck ramps inside its Air Brakes section. What does the notice say?
Correct answer: There is no fine for using a runaway truck ramp
The notice tells truckers they will not be fined for taking a runaway truck ramp. A driver whose brakes cannot hold the vehicle on a grade has no penalty to weigh against using one.
Source: Idaho Commercial Driver's License Manual — Section 5, Air Brakes
20. The manual compares the way an air disc brake grips the disc or rotor to a common shop tool. Which one?
Correct answer: A large c-clamp
When the power screw turns, a caliper squeezes the disc or rotor between its lining pads. The manual likens that action to a large c-clamp.
Source: Idaho Commercial Driver's License Manual — Section 5.1.8, Foundation Brakes