Michigan CDL - Air Brakes Practice Exam

How are spring brakes generally held in place while driving?

1. By hydraulic pressure

2. By mechanical friction

3. By air pressure

4. By electric power

The correct answer is: By air pressure

Spring brakes are typically held in place while driving by air pressure. In an air brake system, the spring brakes are designed to engage automatically when there is a loss of air pressure. Under normal operating conditions, air pressure from the brake system keeps the spring brakes released, allowing the vehicle to move freely. When the driver applies the brakes, air pressure is directed to the brake chambers where it holds the springs back, preventing them from engaging the brakes. Therefore, during normal driving conditions, the presence of air pressure is critical for the functionality and performance of the spring brakes. If there were a failure in the air system or if the pressure were to drop significantly, the springs would engage automatically, applying the brakes and bringing the vehicle to a stop. This design ensures a reliable backup braking system in the case of an air system failure.