What occurs if air pressure drops below 85 psi in the spring parking brake system?

In a spring parking brake system, the brakes are actuated by large coil springs which are held in a disengaged position by air pressure. When the air pressure in the system drops below a critical threshold, typically around 85 psi, the air pressure is not sufficient to keep the brakes released. Consequently, the brakes will automatically engage, which means they will not return to the full off position and will remain applied to prevent the vehicle from rolling or moving unexpectedly.

This safety feature is designed to ensure that if there is a loss of air pressure, which could indicate a failure in the system, the vehicle remains secure, preventing potential accidents. As the pressure continues to drop, the spring force will overcome the reduced air pressure, leading to brake engagement.

The other answer choices do not accurately describe the actions of the spring parking brake system when low air pressure occurs. The vehicle's engine will not shut down, it does not allow for increased speed limits, and brakes will not release automatically under low pressure. Understanding this mechanism is crucial for safety and proper operation of vehicles equipped with air brake systems.