Why must air tanks (rezervoari za zrak) on older or non-automatic pneumatic systems be manually drained of moisture and sludge via drain valves (ventili za odvodnjavanje) regularly?

1. To purge accumulated water condensate and oil residue that can freeze in cold weather and corrode internal brake valves
2. To reduce vehicle total laden weight for weigh stations
3. To replenish the nitrogen concentration in the air compressor
4. To increase engine fuel injection pressure