What keeps spring brakes retracted while the vehicle is in motion?

• A. Hydraulic fluid pressure
• B. Air pressure
• C. Mechanical linkage
• D. Vacuum

Answer: (B)

In a spring brake system, a very strong coil spring sits in the brake chamber and would apply the brakes if the air pressure is removed. Air pressure is used to hold that spring in a retracted position, keeping the brakes released while the vehicle is moving. As long as there is adequate air pressure, the piston is urged in a way that neutralizes the spring’s force. If air pressure is lost, the spring expands and the brakes are applied—this is the fail-safe behavior. Hydraulic fluid pressure, mechanical linkage, or vacuum aren’t responsible for keeping the spring brakes released in this setup, which is why air pressure is the correct answer.