What can legally hold a parking or emergency brake in position for a truck, truck tractor or bus?

What can legally hold a parking or emergency brake in position for a truck, truck tractor or bus?

• A. Hydraulic pressure
• B. Compression springs only
• C. Manual lever
• D. Spring pressure

Answer: (D)

Spring pressure. In heavy-vehicle air brake systems, the parking/emergency brakes use spring brakes. Air pressure is used to release the springs, but when air pressure is removed, the stored energy in the springs pushes the brake shoes against the drums to hold the brakes in the applied position. That spring force is what actually keeps the brake engaged. Hydraulics don’t control this holding action, and the manual lever is a control to operate the system rather than the mechanism that maintains the applied state.

Spring pressure. In heavy-vehicle air brake systems, the parking/emergency brakes use spring brakes. Air pressure is used to release the springs, but when air pressure is removed, the stored energy in the springs pushes the brake shoes against the drums to hold the brakes in the applied position. That spring force is what actually keeps the brake engaged. Hydraulics don’t control this holding action, and the manual lever is a control to operate the system rather than the mechanism that maintains the applied state.