As we have discussed in the past, it is important to service your vehicle’s air dryers to prevent excessive moisture buildup. It is also a good idea to periodically drain the air tanks of any moisture that may have accumulated. Trucks generally have between two and four air tanks. There are three types of drains: automatic (may or may not be heated), cable, and petcock. The automatic drains will purge themselves. The cable and petcock drains need to be manually operated.