During the air brake pre-trip inspection, why must the driver check that spring brakes automatically apply when air pressure falls?

SPRING BRAKE AUTOMATIC APPLICATION TEST is a critical safety check because spring brakes serve as both the parking brake AND the emergency backup braking system. WHAT SPRING BRAKES ARE: Spring brakes work on a fail-safe principle — they are held RELEASED by air pressure (typically 60-90 psi keeps them retracted). When air pressure drops below approximately 20-45 psi, large coil springs expand and mechanically force the brakes on. This means: if the air system fails catastrophically while driving, the brakes apply automatically; the vehicle cannot drive away with spring brakes set unless air pressure is sufficient. PRE-TRIP PROCEDURE TO TEST: With engine off, release spring brakes using the yellow diamond-shaped knob; drain air pressure by repeatedly pressing the foot brake; watch the low air pressure warning — it should activate at or above 60 psi; continue draining pressure; spring brakes should automatically apply (the vehicle becomes difficult or impossible to push) at approximately 20-45 psi; this confirms the spring brake mechanism is functional. WHY THIS MATTERS: A spring brake that fails to set because of a stuck valve or corroded mechanism provides no emergency protection. A driver who discovers the spring brakes don't work during a pre-trip test is in the shop lot, not on the highway — a test failure in the shop is far safer than a system failure at 60 mph.