CDL Air Brakes TestAir Brake System PartsMedium
A driver is performing a pre-trip inspection on a commercial vehicle. When they fully apply the service brakes, they notice that the brake pedal has an excessive amount of free play before the brakes begin to engage. What is the most likely cause of this issue?
- AThe anti-lock braking system (ABS) is malfunctioning.
- BThe air compressor governor is faulty.
- CThe parking brake control valve is stuck.
- DThe slack adjusters require adjustment.
Show answer & explanationAnswer & explanation
Correct answer: D. The slack adjusters require adjustment.
Excessive free play in the brake pedal before the brakes engage is a classic symptom of out-of-adjustment slack adjusters. Slack adjusters compensate for lining wear, and if they are not correctly adjusted, more pedal travel is needed to move the pushrod far enough to apply the brakes.
Why the other options are wrong
- A. ABS issues typically manifest as warning lights, pulsing, or wheel lock-up during braking, not excessive pedal free play.
- B. A faulty air compressor governor would affect air pressure regulation (cut-in/cut-out), not pedal free play.
- C. A stuck parking brake control valve would prevent the parking brakes from releasing or engaging correctly, not affect service brake pedal free play.
Slack Adjuster Function
Slack adjusters automatically or manually adjust the distance between the brake shoe and the drum to compensate for brake lining wear, ensuring proper brake application with minimal pedal travel.
- Maintain correct brake pushrod stroke.
- Prevent excessive brake pedal travel.
- Critical for efficient and safe braking.
Memory trick: Too much pedal travel? The slack's too loose, time to adjust the juice!