40. A driver crosses the air lines when hooking up to an old trailer. What will happen?
Detailed Explanation
The correct answer is "If the trailer has no spring brakes, you could drive away but you wouldn't have trailer brakes.". This question tests combination vehicles knowledge and the safest action expected of a commercial driver. 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: If the trailer has no spring brakes, you could drive away but you wouldn't have trailer brakes.
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 "If the trailer has no spring brakes, you could drive away but you wouldn't have trailer brakes." 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 hand valve will apply the tractor brakes instead of the trailer brakes."The hand valve will apply the tractor brakes instead of the trailer brakes." 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 brake lights will come on when the brake pedal is pressed."The brake lights will come on when the brake pedal is pressed." 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.
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Start Practice TestRelated Questions
20. The driver crosses the air lines when hooking up to an old trailer. What will happen?
96. A driver crosses the airlines when hooking up an old trailer with no spring brakes.
100. What might happen if the trailer is too high when you should line up?
24. After connecting the air lines, but before backing under the trailer you should:
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:
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.