Compressed air usually has some water and compressor oil in it which is bad for the air brake system. For example, the water can freeze in the cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank and can get into the valves and lines if you do not drain the tanks. Summer is the most important time to drain air tanks. Drain a couple times a week from all the air tanks on the truck. Most trucks have multiple tanks. It is also good to try draining them when the truck is on an incline. Let us know if you need help finding them.