When do spring brakes on a tractor or straight truck engage fully?

Answer: B — When air pressure drops to a range of 20-45 psi

The spring brakes on a tractor or straight truck engage fully when the air pressure drops to a range of 20-45 psi. This is a critical safety feature designed to prevent accidents in the event of a loss of air pressure in the braking system. When air pressure falls to this specific range, the spring brakes automatically engage, applying the brakes to help bring the vehicle to a stop. This mechanism ensures that if there is a failure in the air brake system or if the driver loses air pressure, the springs will activate the brakes as a fail-safe, maintaining control of the vehicle. Understanding this function is vital for any driver, as it highlights the importance of monitoring air pressure levels while operating a vehicle equipped with air brakes.