What is a common symptom of air brake system malfunction?

Answer: B — Unusual vibrations while braking

Unusual vibrations while braking can indicate a problem within the air brake system. These vibrations can be a symptom of issues such as uneven brake wear, misalignment, or air pressure problems that affect the performance of the brakes. When the braking action is not smooth or consistent, it may disrupt the normal operation and stability of the vehicle, leading to vibrations that can be felt through the brake pedal or the vehicle itself. Identifying this symptom can help in diagnosing more serious underlying issues that require immediate attention to ensure the safety and reliability of the braking system. In contrast, the other symptoms listed either suggest a normal condition, such as brakes producing no noise or operating consistently, or may indicate different issues, like stiffness which is not directly linked to malfunction in the same way. Recognizing the signs of malfunction, particularly unusual vibrations, is crucial for maintenance and safety.