What must be done if you hear a loud hissing sound in the air brake system?

Hearing a loud hissing sound in the air brake system indicates that there may be a significant air leak. This is critical to address because air brakes rely on a steady and sufficient supply of compressed air to function effectively. If there is a leak, it can lead to a loss of brake pressure, which compromises the vehicle's ability to stop safely.

By immediately checking for air leaks in the system, you can identify the source of the hissing sound and determine whether it can be repaired or if it poses a serious safety risk. Addressing the issue promptly ensures that the air brake system remains fully operational, thereby preventing potential accidents that could occur due to brake failure.

Ignoring the noise or assuming that the brakes are functioning well is dangerous, as a small leak can escalate into a larger problem if left unexamined. Reducing speed and proceeding cautiously may seem prudent, but it doesn't resolve the underlying issue. Similarly, using the emergency brake to stop immediately, while it may provide a temporary solution, does not address the critical need to investigate and fix the leak. Taking immediate action to inspect and repair the air brake system is the safest and most responsible approach.