This is what my system looks like:

http://www.i2.i-2000.com/~rbartick/wilson.jpg

The lower right side of the picture is the water main from the
house. The left side is the water going to the electronic valves.
I was connected my air line to the removable drain cap seen at
the bottom of the PVC on the right side of the picture. This cap
is before the breaker. I also tried to connect my air line to the
small test port on the upper right side of the picture right below
the Wilson breaker. I didn't try to connect air to the small test
port after the breaker on the left side of the picture because I
was worried that air might flow back towards the valve in the wrong
direction and possibly damage it.

I do not have auto drain valves and there are no other ports I can
connect to. I definitely got the heads to pop last year using the
drain plug but I am not sure what I did. I may have disconnected the
50-ft copper coil that sits on my compressor output. I am going to
try to move the compressor right near the sprinkler valve and run a
short line from the compressor to the sprinkler.