At approximately what air pressure do the spring (emergency/parking) brakes typically apply automatically if air pressure is lost?

The SPRING (emergency/parking) BRAKES typically apply AUTOMATICALLY when air pressure drops to between about 20 and 45 PSI (the exact point varies by vehicle). CDL air brakes. This is the FAIL-SAFE: spring brakes are held OFF by air pressure, so when air is lost (a major leak or failure), the springs apply the brakes — stopping the vehicle rather than leaving it with no brakes. SEQUENCE as air drops: low air WARNING at ~60 psi (driver should stop) → if air keeps dropping, spring brakes apply at ~20-45 psi (possibly suddenly). This is why the driver must stop safely when the low-air warning comes on — before the spring brakes apply unexpectedly. Knowing the spring brake automatic application range (~20-45 psi) is commonly tested.