I had a similar problem recently. Thought I had a broken pipe, and since I don't dig anymore, I call my lawn service. Turned out the problem was the pop ups were all very old and were leaking big time every time the zone watered. New pop ups and the problem was solved. Try turning the zone on and watching the pop ups very closely. I'll bet you see water pouring out from around the caps. You don't have to really excavate a big hole to replace the pop up, just a shovel deep cut around the head and lift that much out. This will expose enough of the pop up to allow its removal and for a new one to be screwed on. If I had realized this, I could have save some $$ on the service call.