The spring brakes on trucks will bring it to a stop automatically when air pressure falls to what range?

• A. 0-5 psi
• B. 20-45 psi
• C. 60-80 psi
• D. 100-120 psi

Answer: (B)

Spring brakes are a safety feature that are held off by air pressure and automatically apply when that pressure falls enough. When the system drops into roughly 20 to 45 psi, the springs push the brake shoes against the drums to stop the vehicle. This low-pressure range is chosen so that if the air supply leaks or fails, you’ll come to a controlled stop rather than lose braking abruptly. Pressures higher than that (60–80 psi or 100–120 psi) are still in the normal operating range where the parking brakes are released and the spring force isn’t engaged, so they wouldn’t trigger an automatic stop. Very low pressure like 0–5 psi would indicate the system is already in a very low state, not the standard threshold for automatic spring-brake application.