I found a time delay relay at the scrap yard, along with a solenoid operated valve (I'm not lucky, just persistent). I've got the relay set so it turns the valve on for a fraction of a second whenever the compressor turns on. This works very well, way better than me remembering to walk around the back of the building and drain the tank manually.
One problem with it is in winter the valve can ice up (since it is outside). It tends to ice when the solenoid opens, which means the valve doesn't quite close right and makes a slow leak.
Steve