What is the function of the supply (wet) tank in an air brake system?

The SUPPLY TANK (also called WET TANK or primary reservoir) is the first air tank in the air brake system. It sits immediately downstream from the air compressor and has specific functions:

Receives compressed air first: Air from the compressor enters the supply tank before going anywhere else in the system.

Collects contaminants: The compression process introduces moisture (water vapor that condenses) and small amounts of oil from the compressor. The supply tank provides a large volume where these contaminants can settle rather than flowing into valves and brake chambers throughout the system.

Drain valve: The supply tank has a drain valve at its lowest point specifically for draining accumulated water and oil. The drain valve should be opened daily (or more frequently in high-humidity climates or cold weather) to prevent: corrosion of tank walls; water freezing in lines in cold weather; oil fouling of valves; in dual-circuit systems, water migrating to service brake circuits.

Air dryers: Many modern vehicles have an air dryer placed between the compressor and supply tank — it removes moisture before it enters the tanks, reducing the maintenance burden. Even with an air dryer, drain valves should be checked.

System sequence: Compressor → Supply/Wet Tank → System Protection Valve → Primary and Secondary Tanks → Service Brake Circuits.