The sprinkler system in my house is approx 12 yrs old. It has previously been working fine since I moved in 5 yrs ago. The small front lawn is covered well by two zones. Recently we noticed 1/2 the lawn drying out, correlating exactly with one zone. When observing the sprinklers when on, the pressure appeared very low, and the heads would not pop up in this zone. There was not obvious area in the ground that became wet or flooded. When I pulled up all the heads, they stayed up but with little pressure. They now will pop-up when activated by the timer, but still with 50% pressure. Now, I'm quite a novice with valves, but when I turn the small screw on the valve (and water comes out from the screw hole at a good pressure), the zone will pop up with 100% pressure. In fact, if the zone was activated by the timer it will be 50% pressure, and while running if I turn the screw it will go to 100%. So where is the problem? In the solenoid?