9. After you supply air to the trailer, make sure the air lines are not crossed and The trailer brakes are working. This is done by:
Detailed Explanation
The correct answer is "Applying and releasing the trailer brakes and listening for brake sounds.". This question tests controlled braking, heat management, and maintaining steering control. Because the prompt uses negative wording, identify the normally correct safety rules first, then select the exception.
Correct Answer: Applying and releasing the trailer brakes and listening for brake sounds.
Why This Matters on the CDL Test
This question belongs in your brake safety review. Pay close attention to pressure limits, inspection steps, stopping distance, and warning signs because brake-system mistakes are a common reason CDL applicants miss endorsement questions.
How to Remember It
Say the rule in your own words, then explain why the wrong answers are unsafe or incomplete. That habit makes similar Combination Vehicles questions easier when the exam changes the wording.
Answer Review
Choose "Applying and releasing the trailer brakes and listening for brake sounds." because this answer matches the safe CDL action for brake system safety. On the knowledge test, the safest answer is usually the one that protects space, control, visibility, legal compliance, or inspection accuracy before the vehicle is put at risk.
Why the other answers are weaker
- Lifting the brake pedal."Lifting the brake pedal." does not best answer this brake system safety question. When you see a similar option on the CDL permit test, compare it against the exact safety rule and choose the answer that prevents the most immediate risk.
- Turning on the parking brakes from the cab."Turning on the parking brakes from the cab." does not best answer this brake system safety question. When you see a similar option on the CDL permit test, compare it against the exact safety rule and choose the answer that prevents the most immediate risk.
Test-Day Checklist
- Read the question once for the topic, then read it again for the exact CDL action being tested in Combination Vehicles.
- Eliminate answers that sound convenient but skip an inspection, warning sign, or safety step.
- Say the correct rule out loud before moving on so you can recognize it when the exam changes the wording.
Ready to practice for real?
Take our free, full-length Combination Vehicles practice test. Simulates the actual DMV exam with timed questions.
Start Practice TestRelated Questions
28. You are driving a combination vehicle when the trailer breaks away, pulling apart both air lines. You would expect the trailer brakes to come on and:
45. If the brakes are not released when you pushed the trailer air supply valve you should:
67. You are uncoupling a trailer. After you shut off the trailer air supply and lock the trailer brakes, you should:
98. Charge the trailer air brake system and check that the trailer rolls freely. Then stop and pull out the trailer air supply control valve. Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on.
20. The driver crosses the air lines when hooking up to an old trailer. What will happen?
Keep Studying Combination Vehicles
Question pages help you review one rule at a time. To build test-day speed, move back into the full Combination Vehicles practice test and answer a mixed set without looking at the explanation first.
State CDL Practice Paths
After you understand this answer, use a state practice path to connect the federal CDL rule with the DMV test pages drivers search for most.