Which condition contributes to spongy braking actions due to trapped air in the system?

• A. Worn rotors
• B. Fluid viscosity
• C. Air in the system
• D. Too much fluid

Answer: (C)

Air in the hydraulic system makes brakes feel spongy because air is compressible while the brake fluid is not. When you press the pedal, the pump has to compress any trapped air bubbles before it can build hydraulic pressure to push the pistons against the brake linings. That means part of the pedal travel goes into squeezing air instead of applying braking force, so the pedal goes down farther and the braking response seems soft and delayed. Air gets trapped from leaks, improper bleeding, or maintenance errors, and the cure is to bleed the system properly (and inspect for leaks and fluid quality) so any air bubbles are removed. Worn rotors cause pulsation or vibration rather than a soft, spongy pedal. Changes in fluid viscosity or having too much fluid can affect feel or pressure, but they don’t create the compressible air cushion responsible for sponginess.