To activate emergency brakes (spring brakes), the air pressure must be between:

The correct range for activating emergency brakes, or spring brakes, is between 20 to 45 psi. This pressure range is crucial for ensuring the safety of the bus and its passengers. When the air pressure in the system drops to this level, it indicates that the main brakes may be failing or losing pressure, prompting the emergency brake system to engage automatically.

The importance of this pressure range lies in the purpose of the emergency brakes themselves: they are designed to activate when the normal braking system's air pressure is insufficient to maintain effective brake function. By ensuring that the spring brakes can engage at this level, the system helps prevent potential accidents due to brake failure.

Understanding the operational pressures of emergency brakes is vital for any school bus driver. It allows them to recognize the signs of air pressure loss and take appropriate action before the brakes fail completely.