Parking brakes should engage whenever the rear brake system pressure drops below:

Parking brakes are a crucial safety mechanism in vehicles, especially those used in emergency services like fire trucks, which often rely on air brake systems. When the rear brake system's pressure falls below a certain threshold, there is a potential risk of brake failure, which could lead to a loss of vehicle control.

The correct threshold for when parking brakes should automatically engage is 40 PSI (275 kPa). This ensures that if there is a significant loss of pressure in the brake system, the parking brake will activate, preventing the vehicle from rolling or becoming uncontrollable. Engaging the parking brake at this pressure level provides an additional layer of safety, allowing operators to manage brake system failures more effectively.

Another reason this pressure is significant is that it strikes a balance between maintaining operational functionality under normal conditions while ensuring safety mechanisms activate in a timely manner should the brake system begin to fail. This is especially important in the context of emergency vehicles, where stopping power and control are essential.