During normal driving, spring brakes are usually held back by:

During normal driving, spring brakes are held back by air pressure. This is crucial because the system uses air pressure to keep the springs in a compressed state when the vehicle is in motion. When the driver applies the brakes, air pressure from the braking system pushes against the springs, preventing them from engaging.

In case of an air pressure loss, the springs will automatically engage to bring the vehicle to a stop, which serves as a safety feature. This reliance on air pressure distinguishes spring brakes from other braking systems, which may utilize hydraulic pressure or mechanical components to operate.

The other options do not accurately describe the role of spring brakes during normal driving conditions. Hydraulic pressure is used in standard brake systems, mechanical linkages may be part of some other braking components, and gravity does not play a role in sustaining the spring brakes in a compressed state during operation.