What does the term 'spring brakes' refer to on a heavy vehicle?

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1. An older term for the vehicle's shock absorbers
2. A soft suspension system that absorbs bumps in the road independent of braking, found mainly on older trailers
3. A set of brakes held on by powerful springs, released by air pressure, that apply automatically if air pressure is lost
4. A backup horn that activates when the main horn fails

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