If air pressure falls below 40 psi, what happens to the brakes?

When air pressure drops below 40 psi, the brakes will not operate effectively. This is a critical safety feature in air brake systems, which are commonly used in large vehicles like fire trucks and buses. The threshold of 40 psi acts as a warning limit; below this point, the air pressure is insufficient to keep the brakes released or to apply them properly.

In many air brake systems, low air pressure can lead to a condition where the spring brakes engage automatically as a fail-safe mechanism. This inability to operate correctly at low air pressure ensures that the vehicle does not unintentionally move or become unsafe to control. Thus, this system is designed to prioritize safety, and if air pressure drops below this critical level, the expected operation of the brakes would be compromised, preventing them from functioning normally.