Cut tube 6" back from the valve. Stick a nail in the short stub of tube. This stops outflow from the valve and holds it closed. Now plug the control tube running to the controller in the same manner. Then apply pressure to the system from a garden hose tube adapter attached to the discharge tube of the controller. Index the clock to the station in question. This forces water through the clock out through the tube to the break. The main line shut-off valve need not be closed. Wait for the wet spot(s) to show you the leak.