Air Brake Practice Test

The correct choice indicates that spring brakes have engaged when there is a loss of air pressure. In air brake systems, spring brakes rely on stored air pressure to remain disengaged. When the air pressure drops below a certain threshold, typically due to a failure or when the driver releases the foot brake, the spring brakes automatically engage. This is a safety feature designed to prevent the vehicle from rolling away when there is insufficient air to keep the brakes released.

The other options do not reflect the mechanism by which spring brakes operate. Increased air pressure actually keeps the brakes disengaged, warning lights are generally activated by other indicators (such as low air pressure), and the engagement of the foot brake is not directly related to the spring brakes’ operation but rather to the application of service brakes. Understanding this concept helps to underscore the importance of monitoring air pressure in the braking system to ensure safe vehicle operation.