How do spring brake actuator chambers (Tristop / parking brake chambers) mechanically apply and release the parking/emergency brakes?

1. They require high continuous air pressure to apply the brakes, and release when air is completely vented
2. A powerful mechanical coil spring applies the brakes when air pressure is vented; compressed air pressure is required to compress the spring and release the brakes
3. They rely exclusively on hydraulic fluid pressure generated by an electric motor
4. They are applied by an electromagnet that stays magnetized only while the engine is running