What component automatically applies the brakes to prevent movement when air pressure is low?

The component that automatically applies the brakes to prevent movement when air pressure is low is the emergency brake. In air brake systems, the emergency brake system is designed to engage when there is a significant loss of air pressure, which can occur due to leaks or other malfunctions. This feature ensures that the vehicle remains stationary and minimizes the risk of rolling away when there is insufficient air pressure to operate the regular braking system.

Emergency brakes typically utilize spring pressure to apply the brakes, which is a crucial safety feature, especially in commercial vehicles. This automatic engagement helps protect both the vehicle and any surrounding environment by preventing unintended movement when the air pressure drops below a safe operating threshold.

In contrast, other components like the relay valve, brake pedal, and brake chambers serve specific functions within the air brake system but do not automatically activate the brakes in response to low air pressure.