CDL General Knowledge TestDriving SafelyMedium
A commercial driver is preparing to operate a vehicle equipped with hydraulic brakes. When checking the brake system, the driver notices that the brake pedal goes to the floor with little resistance. What is the most likely cause of this issue?
- AWorn brake pads.
- BOver-adjusted brakes.
- CA faulty brake light switch.
- DAir in the hydraulic lines.
Show answer & explanationAnswer & explanation
Correct answer: D. Air in the hydraulic lines.
A brake pedal that goes to the floor with little resistance is a classic symptom of air in the hydraulic lines. Air is compressible, unlike brake fluid, so it prevents the hydraulic system from building sufficient pressure to actuate the brakes effectively. Over-adjusted brakes or worn pads would typically result in a hard pedal or reduced braking efficiency, not a pedal going to the floor.
Why the other options are wrong
- A. Worn brake pads would increase pedal travel and reduce braking effectiveness, but usually would not cause the pedal to go all the way to the floor with no resistance.
- B. Over-adjusted brakes would typically make the pedal feel very hard and engage quickly with little travel, not cause it to go to the floor.
- C. A faulty brake light switch would only affect the brake lights and would not impact the feel or function of the brake pedal itself.
Spongy Hydraulic Brakes
A condition where a hydraulic brake pedal feels soft or goes to the floor with little resistance, indicating a problem within the hydraulic system.
- Most common cause is air in the hydraulic lines.
- Can also be caused by a fluid leak or master cylinder failure.
- Leads to significantly reduced or complete loss of braking power.
Memory trick: Soft pedal, soft stopping, means air or a leak.