Manually operated by turning a quarter turn (see Figure 5-1) or by pulling a cable. You must drain the tanks yourself at the end of each day of driving. Automatic, in which the water and oil are automatically expelled. They may be equipped for manual draining as well.Click to see full answer. Also question is, why do air tanks on brake systems need to be drained? Draining Air Tanks Any of the reservoirs in a brake system can accumulate moisture, and this moisture can turn to ice when it gets cold or below freezing outside.Also, how do you drain an air tank? Open the drain valve and allow the moisture to drain from the tank. Tilt the tank to remove any remaining moisture. When completely drained close the tank drain valve. Considering this, how often should an air compressor be drained? While strictly speaking you may not need to drain it every time you use the compressor, every couple of days should do. But, you sure do want to drain the tank after every use if you only use the compressor every once in a while. Don’t leave water in the compressor tank to work away at rusting it out.Why do I have so much water in my air tanks?Air includes water, so moisture is always present in your compressed air to varying degrees depending on humidity and temperature. An accumulation of water can cause moisture to get pulled back into the compressed air stream.