Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of:

Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of:

• A. 60 to 80 psi
• B. 10 to 15 psi
• C. 45 to 60 psi
• D. 20 to 45 psi

Answer: (D)

Spring brakes act as a fail-safe parking brake: they’re held off by the system’s air pressure, and if that pressure drops enough, the springs force the brakes on to stop the vehicle. For tractors and straight trucks, the point at which the springs fully engage is generally in the 20 to 45 psi range. When the pressure falls into roughly this window, the springs release their hold and the brakes come on fully to stop the vehicle. Higher pressures, around 60–80 psi, keep the parking brakes released, while pressures as low as 10–15 psi are well below the normal engagement window and indicate the system is already in a dangerous state. So the correct range for full spring-brake activation is 20 to 45 psi.

Spring brakes act as a fail-safe parking brake: they’re held off by the system’s air pressure, and if that pressure drops enough, the springs force the brakes on to stop the vehicle. For tractors and straight trucks, the point at which the springs fully engage is generally in the 20 to 45 psi range. When the pressure falls into roughly this window, the springs release their hold and the brakes come on fully to stop the vehicle. Higher pressures, around 60–80 psi, keep the parking brakes released, while pressures as low as 10–15 psi are well below the normal engagement window and indicate the system is already in a dangerous state. So the correct range for full spring-brake activation is 20 to 45 psi.