CDL General Knowledge TestDriving SafelyMedium
A commercial driver is driving a vehicle equipped with hydraulic brakes and notices that the brake pedal feels 'spongy' or goes nearly to the floor before engaging. What is the most likely cause of this symptom?
- AAir in the hydraulic brake lines.
- BWorn brake linings in the wheel ends.
- CA faulty air compressor.
- DOver-adjusted slack adjusters on the brake drums.
Show answer & explanationAnswer & explanation
Correct answer: A. Air in the hydraulic brake lines.
Air in the hydraulic brake lines is the most common cause of a 'spongy' pedal. Air is compressible, unlike brake fluid, so the pedal must travel further to build pressure, giving a soft or spongy feel.
Why the other options are wrong
- B. Worn brake linings would typically cause a hard pedal or reduced braking, not a spongy feel unless accompanied by other issues.
- C. A faulty air compressor affects air brakes, not hydraulic systems.
- D. Slack adjusters are for air brakes; they don't apply to hydraulic systems.
Spongy Hydraulic Brakes
A condition where a hydraulic brake pedal feels soft or sinks to the floor before the brakes engage, indicating a problem in the hydraulic system.
- Most often caused by air in the brake lines.
- Can also be caused by leaking master cylinder or worn components.
- Requires immediate inspection and repair, typically bleeding the brake lines.
Memory trick: Spongy pedal? Air in the channel!