Was looking at the air tank valves on my Volvo 610, the middle and rear tank have screw in drain valves the front tank(wet tank) has a different looking valve with a cable hanging from it. wondering why it is different? Drained the middle and rear tank not much came out ( system had very little air in it) unthreaded the valve out of the front tank and got about a quart of oil out of it, is that normal? I have owned the truck for about a year and I am trying to learn the systems, I am assuming this needs to be drained a lot more often. I have only put on about 1000 miles since I bought it Thanks for any info Stan