I don't think you have enough CFM thru your compressor. I put in a sprinkler system in my last house (no time here yet), and put a low pressure drain in each zone at the lowest point. I also put a valve for a compressor, but the drains did the trick and never had to worry about them other than shutting off the water supply and opening the valves so the water could go to the lowest point.

I would say to turn off the water and open 1 valve, blast the air and see if any water came out. Close that one and open another, repeat. If your not getting anything and worried, borrow a larger compressor and blast it.

If your really worried, pull a valve and see if water runs out (only if they are burried, if above ground disregard). For some reason, a lot of people have the valves really low and water sits there. If that is the case, and you pull a valve all the water will run out of the line.

There are many things you can do, just depends on how much time your willing to devote. These are the most simple, good luck.

Jeff