What occurs at 150 psi pressure in an air brake system?

In an air brake system, when the pressure reaches 150 psi, the safety valve opens to release excess air. This is a crucial safety feature designed to prevent over-pressurization within the brake system. If the pressure exceeds the system's safe operating limits, the safety valve automatically releases air to maintain the pressure within a safe range. This helps prevent potential damage to components of the air brake system and ensures continuous safe operation.

The other options relate to different actions within the braking system. For instance, the service brake engages at a much lower pressure, while the parking brake and emergency brake each have specific operational requirements and conditions that dictate their engagement. Understanding how the safety valve functions reinforces the importance of the pressure limits that are essential for safe driving and vehicle operation.