93. The trailer air supply control (also known as the tractor protection valve control) should automatically close when the air pressure falls to between:
Detailed Explanation
The correct answer is "c. 20 and 45 psi.". This is based on federal CDL regulations and safety standards.
Correct Answer: c. 20 and 45 psi.
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 "c. 20 and 45 psi." 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
- a. 60 and 80 psi."a. 60 and 80 psi." 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.
- b. 45 and 60 psi."b. 45 and 60 psi." 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.