When should the spring brakes activate?

Check that spring brakes come on automatically: Chock the wheels, release the parking brakes when you have enough air pressure to do it and shut the engine off. Step on and off the brake pedal to reduce the air tank pressure. The 'parking brake' knob should pop out when the air pressure falls to the manufacturer's specification, usually in a range between 20 and 40 psi. This causes the spring brakes to come on.