During a pre-trip inspection of a tractor-trailer equipped with a dual air brake system, you discover that the service brake application on the trailer is significantly weaker than on the tractor. Which of the following is the MOST likely cause?

A severely restricted air supply line to the trailer.. A restricted air supply line to the trailer will reduce the air pressure reaching the trailer's brake chambers, resulting in weaker trailer braking compared to the tractor. Excessive slack adjuster travel would cause poor braking on the trailer but might also affect the tractor if it were a shared issue or if the system were severely out of adjustment. Leaking diaphragms in the tractor would weaken tractor brakes. The trailer brake light switch is unrelated to brake application force.

A commercial vehicle's air brake system is equipped with a governor that operates between 95 PSI and 120 PSI. If the vehicle is parked on a steep grade and the driver needs to use the parking brakes, what is the MINIMUM air pressure required in the tank to ensure adequate holding power for the spring brakes?

95 PSI. The governor's cut-in pressure is 95 PSI. This is the minimum pressure at which the compressor begins to build air. While higher pressure is always better for holding power, 95 PSI is the lowest operational pressure where the system is functioning as intended to maintain air. Spring brakes apply when air pressure is released from the brake chambers, but their effectiveness is still dependent on the overall system pressure being adequate.

You are performing a leak test on a tractor-trailer's air brake system. After shutting off the engine and releasing the parking brakes, you note an initial air pressure of 100 PSI. The system should not lose more than 2 PSI in the first minute and 4 PSI in the second minute. If the gauge drops to 97 PSI after one minute and 93 PSI after two minutes, what is the MOST accurate assessment of the system's air loss?

The system has excessive air loss, indicating a significant leak.. The system lost 3 PSI in the first minute (100 to 97) and an additional 4 PSI in the second minute (97 to 93), totaling 7 PSI loss over two minutes. Standard acceptable limits are typically no more than 2 PSI in the first minute and 4 PSI total in the second minute (meaning a maximum of 6 PSI loss over two minutes). This vehicle's loss of 7 PSI exceeds the standard, indicating a significant leak.

Free CDL Air Brakes Test Practice Test

Realistic 25-question practice exam with instant feedback and score reports.

Start free practice test

About this practice exam

Free CDL Air Brakes Test practice test with 25 realistic multiple-choice questions, instant grading, and explanations. Study with drill mode, category score reports, and a personalized review plan.

Exam format

25 multiple-choice practice questions
Drill mode with instant feedback and explanations after each answer

Study tips

Review the official exam content outline before your first practice run.
Take the full practice exam once to establish a baseline score by category.
Focus review on categories where you score below the passing threshold.
Re-take missed questions in drill mode until you can explain each correct answer.
Schedule the real exam only after consistent passing scores on practice tests.

Sample CDL Air Brakes practice questions

Try a few representative questions below. Each includes the correct answer and a short explanation — the same style you'll see in the full practice test.

Question 1Pre-Trip Inspection
During a pre-trip inspection of a tractor-trailer equipped with a dual air brake system, you discover that the service brake application on the trailer is significantly weaker than on the tractor. Which of the following is the MOST likely cause?
- A.A severely restricted air supply line to the trailer.(Correct)
- B.A malfunctioning trailer brake light switch.
- C.A leaking service brake diaphragm in the tractor's brake chambers.
- D.Excessive slack adjuster travel on the trailer's brake chambers.
Explanation
A restricted air supply line to the trailer will reduce the air pressure reaching the trailer's brake chambers, resulting in weaker trailer braking compared to the tractor. Excessive slack adjuster travel would cause poor braking on the trailer but might also affect the tractor if it were a shared issue or if the system were severely out of adjustment. Leaking diaphragms in the tractor would weaken tractor brakes. The trailer brake light switch is unrelated to brake application force.
Question 2Air System Operation
A commercial vehicle's air brake system is equipped with a governor that operates between 95 PSI and 120 PSI. If the vehicle is parked on a steep grade and the driver needs to use the parking brakes, what is the MINIMUM air pressure required in the tank to ensure adequate holding power for the spring brakes?
- A.120 PSI
- B.95 PSI(Correct)
- C.60 PSI
- D.The pressure is irrelevant; spring brakes apply regardless of tank pressure.
Explanation
The governor's cut-in pressure is 95 PSI. This is the minimum pressure at which the compressor begins to build air. While higher pressure is always better for holding power, 95 PSI is the lowest operational pressure where the system is functioning as intended to maintain air. Spring brakes apply when air pressure is released from the brake chambers, but their effectiveness is still dependent on the overall system pressure being adequate.
Question 3Leak Testing
You are performing a leak test on a tractor-trailer's air brake system. After shutting off the engine and releasing the parking brakes, you note an initial air pressure of 100 PSI. The system should not lose more than 2 PSI in the first minute and 4 PSI in the second minute. If the gauge drops to 97 PSI after one minute and 93 PSI after two minutes, what is the MOST accurate assessment of the system's air loss?
- A.The system is only leaking from the trailer, not the tractor.
- B.The system is within acceptable limits.
- C.The system has a minor leak, but it is acceptable for operation.
- D.The system has excessive air loss, indicating a significant leak.(Correct)
Explanation
The system lost 3 PSI in the first minute (100 to 97) and an additional 4 PSI in the second minute (97 to 93), totaling 7 PSI loss over two minutes. Standard acceptable limits are typically no more than 2 PSI in the first minute and 4 PSI total in the second minute (meaning a maximum of 6 PSI loss over two minutes). This vehicle's loss of 7 PSI exceeds the standard, indicating a significant leak.

Want the full exam? Start the free practice test →

Frequently asked questions

How many questions are on this CDL Air Brakes practice test?

This practice test includes 25 multiple-choice questions designed to mirror the format and difficulty of the real CDL Air Brakes Test.

Is this CDL Air Brakes practice test free?

Yes. You can start practicing for free. Create an account to save progress, track weak categories, and retake the exam.

Do I get explanations after each question?

Yes. In drill mode you see why the correct answer is right, why distractors are wrong, and practical examples where relevant.

How should I use this practice test to prepare?

Take the full exam under timed conditions, review missed questions by category, then focus study on your weakest sections before scheduling the real exam.

Does this replace official exam materials?

No. Use this as a supplement alongside official candidate information bulletins, textbooks, and hands-on experience required for your license or certification.

Related practice tests

Ready to test your knowledge?

Free 25-question practice exam — no sign-up required to try.

Start practice test