On a truck equipped with air brakes, the parking brakes are held on (applied) by what mechanism?

• A. Springs
• B. Air pressure
• C. Hydraulic pressure
• D. Mechanical linkage

Answer: (A)

In air brake systems for heavy trucks, the parking (emergency) brakes are spring-applied and air-released. A strong compression spring inside the spring brake chamber tends to push the brake into the applied position. When you set the parking brake, compressed air is used to hold that spring back and keep the brakes off. If air pressure is lost, the spring expands and automatically applies the parking brakes, making it a fail-safe design. So the mechanism that holds the parking brakes on is the spring. Air pressure’s role is to keep the brakes released, not held on.