CDL General Knowledge TestAir Brakes (if applicable)Hard
A driver is performing a pre-trip inspection. When checking the slack adjusters on the S-cam air brakes, they measure the pushrod travel. What is the maximum allowable pushrod travel for most commercial vehicles before adjustment is required?
- A1 inch
- B2 inches
- C1/2 inch
- D1 1/2 inches
Show answer & explanationAnswer & explanation
Correct answer: B. 2 inches
While 1 inch of pushrod travel is often cited as the ideal, the maximum allowable pushrod travel before adjustment is legally required for most commercial vehicles with S-cam brakes is 2 inches (or 1 3/4 inches for some older systems, but 2 inches is the common maximum for out-of-adjustment). Anything beyond 2 inches indicates dangerously excessive travel, significantly reducing braking effectiveness.
Why the other options are wrong
- A. This is often the target for proper adjustment, but not the maximum allowable before adjustment is required.
- C. This is too little travel, indicating a properly adjusted or over-adjusted brake.
- D. This is getting close to the limit but 2 inches is the commonly accepted maximum before requiring adjustment.
Max Pushrod Travel
The maximum allowable pushrod travel for most commercial vehicles with S-cam air brakes before adjustment is required is 2 inches.
- Measured with full brake application.
- Exceeding 2 inches is a critical defect.
- Shorter travel means more effective braking.
- Check all wheels.
Memory trick: Two inches (or 1 3/4) is the max, beyond that, your brakes are lax.