CDL General Knowledge TestAir Brakes (if applicable)Hard
During a pre-trip inspection, a driver checks the brake linings on a vehicle with air brakes. Which of the following conditions would indicate that the brake linings need to be replaced?
- AThere is a small amount of dust visible on the brake drum.
- BThe brake drum has minor scoring marks.
- CThe linings are slightly discolored from normal use.
- DThe lining thickness is less than 1/4 inch at its thinnest point.
Show answer & explanationAnswer & explanation
Correct answer: D. The lining thickness is less than 1/4 inch at its thinnest point.
Brake linings are a critical wear item. If the lining thickness is less than 1/4 inch at its thinnest point (or other manufacturer specifications, but 1/4 inch is a common minimum for replacement), it indicates that the lining is worn out and needs to be replaced to ensure safe and effective braking.
Why the other options are wrong
- A. A small amount of dust is a normal byproduct of brake operation.
- B. Minor scoring on the brake drum may require machining, but it's the lining thickness that dictates lining replacement.
- C. Slight discoloration is normal and does not necessarily indicate a problem.
Brake Lining Wear Limit
Brake linings must be replaced if their thickness is less than 1/4 inch at the thinnest point, or if contaminated with oil/grease.
- Minimum 1/4 inch thickness.
- Check at thinnest point.
- Contamination also requires replacement.
- Worn linings reduce braking power.
Memory trick: Quarter inch, that's the line, less than that, you're out of time.