What symptom indicates air in the brake hydraulic system?

When air enters the brake hydraulic system, it can compress and create a spongy feeling in the brake pedal. This occurs because the air bubbles in the fluid allow for increased compression under foot pressure, resulting in a brake pedal that feels low and spongy. When air is present, it disrupts the hydraulic pressure that is needed for effective braking, which is why a driver may notice the pedal sinking further than usual or feeling less responsive.

In contrast, other symptoms like a grinding noise typically indicate issues with brake components such as worn brake pads or rotors, rather than air in the hydraulic system. A vibrating brake pedal is often associated with warped rotors or problems with wheel alignment. Recognizing the specific symptom of a low and spongy brake pedal is essential for diagnosing the presence of air in the brake system and taking appropriate steps to bleed the brakes and restore proper hydraulic function.