Michigan CDL - Air Brakes Practice Exam

When do spring brakes activate in an air brake system?

1. When air pressure is high

2. When too much air pressure is lost

3. When the brakes are applied hard

4. When the vehicle is stationary

The correct answer is: When too much air pressure is lost

Spring brakes are a crucial safety feature in an air brake system that provide a method for stopping the vehicle in the event of air pressure loss. They are designed to engage automatically when the air pressure in the system falls below a certain level, which is typically around 60 psi. This loss of pressure is often due to a failure in the air brake system, making it imperative that the brakes engage to prevent a runaway vehicle. When the air pressure is sufficient, the spring brakes remain released, allowing normal brake operation. However, if there is a significant drop in air pressure, the spring brakes will activate, using the stored energy in the springs to apply braking force. This function is vital for ensuring safety, as it helps bring the vehicle to a stop even if the primary braking system fails. In contrast, the other options do not accurately describe conditions under which the spring brakes would activate. High air pressure indicates that the system is functioning properly and the brakes would not engage. Simply applying the brakes hard or being stationary does not result in spring brake activation; it is specifically the loss of air pressure that triggers this safety mechanism.