Final answer:
Spring brakes in vehicles with air brake systems, such as tractors and straight trucks, will fully engage when the air pressure drops to a range of 20 to 45 psi as a safety measure.
Step-by-step explanation:
Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi. Spring brakes are an essential component of an air brake system which is commonly used in heavy vehicles such as tractor-trailers and buses. The air brake system utilizes compressed air to apply pressure to pistons in the brake chamber, which in turn apply pressure to the brake pads or shoes, creating the friction needed to slow down or stop the vehicle.
In case of air pressure loss, the spring brakes, which are held in retract position by air pressure during normal operation, will deploy to prevent the vehicle from moving. This is a safety measure that ensures the vehicle can come to a stop even if there's a failure in the air pressure system. Thus, the correct answer to when the spring brakes will fully engage due to low air pressure is within the range of 20 to 45 psi.