What do spring brakes do?

Spring brakes are designed to engage automatically when the air pressure in the system drops to a predetermined level, which typically happens when air has been released or when there is a failure in the air pressure system. This mechanism serves as a safety feature to ensure that the vehicle remains stationary and does not roll away in the event of a loss of air pressure, thereby preventing potential accidents.

The spring brake system uses large coil springs that are compressed by air pressure during normal operation. If the air pressure falls below a certain threshold, these springs are released, forcing the brake shoes against the brake drum, which brings the vehicle to a stop. This is particularly important for heavy commercial vehicles, which rely on air brakes, to maintain safety on the road.

In contrast, the other options do not accurately describe the fundamental purpose of spring brakes. They do not engage when air pressure is too high, nor do they specifically prevent downhill movement as their primary function; their primary focus is on safety related to low air pressure situations. They also do not enhance braking on wet surfaces, as that pertains to different braking technologies or techniques.