- A Connect the electrical line first
- B Drive away immediately
- C Charge the trailer brakes by pushing in the trailer air supply knob Correct answer
- D Disconnect them again
Why this is the correct answer
Push the trailer air supply (red eight-sided knob) in to send air to the trailer and release its spring brakes — but only after a successful coupling.
How this topic appears on the test
The Combination Vehicles CDL exam covers Coupling as a recurring theme. Test-writers favor questions that probe whether the applicant has internalized the safe operating procedure, not merely memorized the rule. When you encounter a question on this topic, slow down and verify that the answer you've chosen describes what a careful, defensive driver would actually do — not what is technically permissible.
If you missed this question, that's exactly the right outcome of a practice session: the goal is to surface gaps before the actual test. Open your state CDL manual to the Coupling section and re-read the relevant subchapter. Then come back and run through the rest of the Combination Vehicles practice exam.
For drivers preparing for the Combination Vehicles CDL exam, additional context — including federal manual excerpts, employer hiring practices, and DOT medical guidance — is widely available from industry resources. Continue reading on a recommended industry resource for further detail. Always cross-check anything you read with the current edition of your state CDL manual, since enforcement guidance is updated periodically.