How does the air dryer purge cycle operate when the compressor governor reaches its cut-out pressure?

1. It rapidly vents a small burst of clean, dry air backward through the desiccant to exhaust collected moisture and contaminants through the purge valve
2. It discharges all reservoir pressure completely to zero bar
3. It circulates hot engine coolant directly through the air brake lines
4. It shuts down the engine automatically to cool the compressor