In a braking system, what can cause a spongy brake pedal?

A spongy brake pedal is typically caused by air being trapped in the hydraulic braking system. When air enters the brake lines, it compresses more easily than the brake fluid, resulting in a soft or spongy feel when the brake pedal is pressed. This is because the presence of air creates non-fluid compressibility in the hydraulic system, preventing the force from being transmitted effectively through the brake lines to the braking components.

Pump pressure is essential for proper brake function, and when the system is filled with fluid rather than air, the braking action is firm and responsive. Conversely, if air is present, the brake pedal may feel mushy or require more travel before resistance occurs. Therefore, bleeding the brakes to remove any trapped air is a critical step in resolving this issue and restoring the pedal's firmness.

The other options referenced do not relate to the performance of the braking system directly, making them irrelevant to the symptom of a spongy brake pedal.