What is indicated by a hissing sound in the air brake system?

Answer: C — Possibly an air leak in the braking system

A hissing sound in the air brake system typically signifies that there may be an air leak within the braking system. This sound indicates that air is escaping from a component, which can compromise the system's ability to build and maintain the necessary pressure for proper brake operation. In air brake systems, maintaining adequate pressure is crucial for ensuring that the brakes function effectively. If there is an air leak, it can lead to insufficient pressure, resulting in reduced braking performance or potential brake failure. Identifying and addressing any hissing sounds promptly is vital to ensure that the braking system remains safe and reliable. Understanding this function helps drivers and inspectors recognize potential maintenance issues before they escalate into more serious problems, making it an important aspect of air brake operation and safety.