hi numerich,

I am not familiar with systems that have auto drain valves, but I would assume they are at the low end of the system. The system usually drains to the lowest irrigation head. You can check for any fitting coming out of the mainline lower than the lowest sprinkler. If you have a system that has sprinklers at the same elevation, look for a low area that a pipe can drain to and would be wet after you use the system. I am not a fan of a system that empties after each use, so you may need to apply air pressure anyway to make the drain operated. Hope this helps with some possibilities. GL. Aloha

Check with the origonal installer. If in doubt blow it out.