What occurs when air pressure in the braking system falls below 60 p.s.i.?

When air pressure in the braking system falls below 60 p.s.i., it may cause the spring brakes to start to drag. This is a safety feature designed to ensure that if the air pressure drops to a dangerously low level, the spring brakes—mechanical brakes that can be engaged by a spring—will automatically engage to help slow down or stop the vehicle. This helps prevent total brake failure by providing an emergency braking function, thus allowing a vehicle to be brought to a stop safely even in the event of an air supply failure.

The spring brake mechanism is vital in heavy-duty vehicles because it serves as a fail-safe to ensure that even in a compromised system, there are still some braking capabilities available, mitigating the risk of accidents that could arise from complete brake failure.