When checking the spring brakes activate at the correct pressure, what should happen?

To check spring brake activation: continue bleeding air out of the system (engine off, key on, brake pedal pumped) past the point where the low-air warning activated. The parking brake control should pop out automatically (or the brakes should otherwise apply) when air pressure drops to between 20 and 45 psi. This 'spring brake' activation is the system's fail-safe — even if air is completely lost, the springs will apply the brakes mechanically. If the spring brakes do not apply automatically before pressure reaches 20 psi, or if they apply too early (above 45 psi), the system has a problem. After confirming the spring brakes engage, restart the engine and let pressure build back up before driving.