A car has a spongy pedal. Which of the following could be the cause?

When a car has a spongy pedal, it often indicates that there is air trapped in the brake system. The presence of air reduces the effectiveness of hydraulic pressure, which is essential for proper braking function. When you press the brake pedal, the expected firm response is diminished due to the compressibility of air, resulting in that "spongy" feeling.

This phenomenon occurs because brake systems operate on hydraulic principles, where brake fluid transmits force from the pedal to the brake components. If air enters the system, it can create gaps that interfere with the hydraulic force transmission. Therefore, removing the air through bleeding the brake system is crucial to restore the proper pedal feel and braking performance.

Worn brake pads could affect brake efficiency but would not typically cause a spongy feel. A faulty brake booster could lead to difficulties in applying the brakes, but it does not directly relate to the sensation of a spongy pedal. Low brake fluid levels can also lead to brake issues, but if the fluid is sufficiently low to cause a spongy pedal, it may instead cause a complete loss of braking ability. Thus, the presence of air in the system is the most accurate cause of a spongy brake pedal.