I have an automatic lawn sprinkler issue. What I noticed the other day is that I have a slow water leak through one row of sprinkler heads. At the area where the valves are located, you can hear a low hiss from one of the valves. I replaced a solenoid at the station that sounded like the hiss was coming from, but the slow leak continues.

I did a lot of work on sprinkler systems in SoCA and I think you are on the right track. The problem I noted most was caused by the diaphragm in the valve leaking. You can get repair kits for most common valve brands at a home center. All it takes to replace the diaphragm in those I worked on was a Phillips screwdriver. Do turn off the water source first! Very seldom did I have to replace the solenoid; they either work or not.

I didn't know they have repair kits, i thought you have to replace the whole valve.
Thanks