What is the most common cause of loss of effective braking in an air brake system?

The most common cause of loss of effective braking in an air brake system is a leak in the air brake system. When there is a leak, it results in a loss of air pressure that is essential for the operation of the air brakes. The air compressor generates air pressure, which is stored in tanks and is used to apply the brakes. If there is a leak anywhere in the system, whether in the air lines, fittings, or brake chambers, it can significantly reduce the air pressure available to activate the brakes. This loss of pressure means that the brakes may not engage fully or at all, leading to a failure to stop the vehicle effectively.

In contrast, while a malfunctioning air compressor can lead to problems in the braking system, it is not the most common issue; a malfunction would need to be severe enough to prevent air generation altogether. Excessive brake lining wear can diminish braking power but typically takes a longer time to develop and may be caught during regular maintenance checks. Overheating of the brake drums, although serious, is often a result of other underlying issues or improper usage and is not as frequently encountered as leaks. Thus, the pervasiveness and critical nature of air leaks in maintaining effective braking make them the most common issue.