Spring brakes will engage when the air pressure drops to which range?

• A. 50-75 psi
• B. 0-10 psi
• C. 20-45 psi
• D. 10-20 psi

Answer: (C)

Spring brakes are spring-applied, air-released devices. They stay released as long as there’s enough air pressure in the system. When that pressure falls to a low threshold, the internal springs snap the brakes on to hold or stop the vehicle. The typical engagement range is about twenty to forty-five psi, which gives a safe, predictable point at which the parking/emergency brake applies as the system bleeds down. If the pressure stays above this range, the springs remain released; if it drops into this window, the springs engage to prevent uncontrolled motion. So, the range around twenty to forty-five psi is the design point at which spring brakes come on.