18. You have a major leak in the service line and you put on the brakes. Service air pressure will escape and cause the:
Detailed Explanation
The correct answer is "Trailer emergency brakes to come on.". This question tests controlled braking, heat management, and maintaining steering control. For a differently worded version of this question, identify the hazard first and then choose the answer that preserves control, visibility, or legal compliance.
Correct Answer: Trailer emergency brakes to come on.
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 "Trailer emergency brakes to come on." 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
- Trailer tank pressure to be lost."Trailer tank pressure to be lost." 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.
- Tractor spring brakes to lock on."Tractor spring brakes to lock on." 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
43. Your emergency air line breaks or gets pulled apart while you are driving. The loss of pressure will cause the:
60. The tractor protection valve will close and the trailer emergency brakes will come on when there is a major leak in the________ brake line.
To test the trailer service brakes, you should:
12. If the service air line comes apart while you are driving a combination vehicle but the emergency line stays together, what will happen right away?
13. The air leakage rate for a combination vehicle (engine off, brakes on) should not be more than _ psi per minute.
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.