Michigan CDL - Air Brakes Practice Exam

What happens if the air pressure drops too low in a vehicle while driving?

1. The service brakes are applied

2. The parking brake engages automatically

3. The spring brakes will activate

4. The vehicle speed decreases

The correct answer is: The spring brakes will activate

When the air pressure drops too low in a vehicle equipped with air brakes, the spring brakes will activate. This is a crucial safety feature designed to prevent accidents caused by loss of air pressure. In normal operation, the air brake system relies on compressed air to hold the brakes off. If the air pressure decreases below a certain threshold, usually around 60 psi, the spring brakes are released and will automatically engage to bring the vehicle to a stop. This ensures that even in the event of an air leak or other malfunction that causes a drop in air pressure, the vehicle can be safely stopped without relying solely on the service brakes. The activation of the spring brakes is a protective measure to help maintain control of the vehicle and prevent it from rolling freely if the air pressure fails, providing a critical layer of safety for both the driver and others on the road. The other options do not accurately reflect the mechanism by which air brake systems ensure vehicular safety under low pressure conditions.