What is the function of the spring brake chambers (frenos de resorte / maxi-brakes) in a heavy vehicle's air brake system?

1. To spray air onto the tires to cool them
2. To provide parking and emergency braking: powerful mechanical springs mechanically apply the rear brakes if air pressure is lost or when the parking brake valve is engaged, requiring pneumatic pressure to release them
3. To soften suspension bounce when driving over potholes
4. To increase engine horsepower at high RPM