- A 5 seconds
- B Half a second Correct answer
- C 2 seconds
- D 1 second
Why this is the correct answer
Air must travel from the foot valve through the lines to the chambers — a delay of roughly half a second on a typical heavy truck.
How this topic appears on the test
The Air Brakes CDL exam covers Brake lag 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 Brake lag section and re-read the relevant subchapter. Then come back and run through the rest of the Air Brakes practice exam.