Determined to find the backflow box, we dug up most of the yard, from the water line at the street all the way to the house.
Notice the oak tree in the picture above? This oak tree was undoubtedly much smaller 25 years ago, and some shortsighted individual decided to install the backflow box right next to the tree. Over the course of time, the tree had grown over the box and crushed it with its roots, so it was a small miracle that we found it. The old box was in such bad condition that we abandoned it and installed a new box, a safe distance from the tree, and rerouted the main water line.
We turned the water on and crossed our fingers. A few sprinkler heads popped up but there was hardly any water pressure. It turned out that all the water was leaking from a busted pipe...which was under our deck. So, of course, we had to remove half of the boards from our deck.
Fast forward a few days. The broken pipe is replaced and we're ready to test the system again. We knew the sprinkler system had four zones because they were labeled in the control box, but water was only running to the first zone. We were left scratching our heads as to how to find the valve boxes, 4 in all, that controlled the water flow to each of the zones. First we thought a metal detector might do the trick, but after some online research, we decided to rent a wire and valve locator. You can find these at irrigation supply or tool rental stores.