What is the primary function of a safety relief valve installed in an air brake reservoir?

1. To automatically vent excess air pressure if the governor fails, preventing the air tanks from rupturing (typically at ~150 PSI / 10 bar)
2. To hold the reservoir at a constant pressure by bleeding air continuously while driving
3. To drain accumulated water from the reservoir automatically at each stop
4. To cut the compressor out once the reservoirs reach working pressure