What happens to the parking (spring) brakes when the vehicle's air pressure drops to a critically low level, even if the driver has not manually set the parking brake?

Correct answer to question 125 (Air Brakes): They apply automatically to prevent driving with insufficient braking capacity

Why

Spring brakes are held released by air pressure, so when pressure drops to a critically low level, strong springs automatically apply the brakes, protecting against driving with unsafe braking capacity.