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:
Detailed Explanation
The correct answer is "The tractor protection valve to close.". This is based on federal CDL regulations and safety standards.
Correct Answer: The tractor protection valve to close.
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 "The tractor protection valve to close." 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
- The trailer supply valve to stay open."The trailer supply valve to stay open." 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.
- The tractor to lose all air pressure."The tractor to lose all air pressure." 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
1. You are coupling a semi trailer to your tractor but have not yet backed under. The trailer is at the right height when:
10. There are two things that a driver can do to prevent a roll over. They are: (1) Keep the cargo as close to the ground as possible; and (2):
100. What might happen if the trailer is too high when you should line up?
101. There are two things that a driver can do to prevent a rollover. They are (1) go slowly around turns; (2) keep:
11. Before you back under a trailer, make sure the:
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.