CDL Combination Vehicles TestDriving Combination Vehicles SafelyHard
A commercial driver is conducting a pre-trip inspection of a combination vehicle. The driver notices that the brake adjustment on one of the trailer's slack adjusters allows more than 2 inches of movement when pulled by hand. What action should the driver take?
- AManually adjust the slack adjuster until the movement is within tolerance.
- BContinue the inspection, as this is a minor issue that will self-adjust.
- CProceed with the trip, but plan to adjust the brakes at the next available stop.
- DTag the vehicle out of service and have a qualified mechanic repair the brake.
Show answer & explanationAnswer & explanation
Correct answer: D. Tag the vehicle out of service and have a qualified mechanic repair the brake.
Excessive slack adjuster movement (typically more than 2 inches for most vehicles) indicates misadjusted brakes, which can severely compromise braking effectiveness and is a critical safety violation. The vehicle must be taken out of service and repaired by a qualified mechanic before operation.
Why the other options are wrong
- A. While manual adjustment can correct it, if it's found during a pre-trip, it's often a sign of underlying issues or neglect. Many modern systems are self-adjusting, and manual adjustment should only be done by qualified personnel after a diagnosis.
- B. This is a significant safety issue, not minor, and will not self-adjust if it's already beyond limits, especially if it's an older manual slack adjuster.
- C. Operating with misadjusted brakes is unsafe and illegal; air brakes require proper adjustment for safe operation.
Brake Slack Adjustment
Excessive slack adjuster movement (e.g., >2 inches) indicates misadjusted brakes, requiring the vehicle to be taken out of service for repair by a qualified mechanic.
- Slack adjusters control brake shoe-to-drum distance.
- Excessive movement reduces braking force and causes uneven braking.
- A major safety defect requiring immediate repair.
Memory trick: Slack's too much, don't you touch! Out of service, that's the clutch!