Question
QUESTION: Matt,
We have a 3 year old Hunter Sprinkler system.  In 3 out of the 12 zones, the heads leak after the solenoid valve has shut off.  I have changed 1 complete valve and it fixed the problem for 7 days.  The leak returned.  What could be causing this problem and how can I fix it?  I would like to fix the problem before turning the sprinkler system back on for the summer.  Thank you for any advice you can offer.

ANSWER: Hello. Sorry for not responding quicker, I'm actually on vacation.

It sounds like you are getting some sort of debris in the valve or valves. When a valve weeps water, it is generally a particle of something holding the diaphragm open. I would remove the top of the valve and flush, flush, flush. Watch for any small contamination pieces. Some of the early Hunter valve were very sensitive to contaminants. It maybe necessary to install a filter to help your problem. A simple course mess filter would work. 30 or 40 mesh would clean the water enough without being a maintenance problem.
If you need recommendations for a filter, I can help.

---------- FOLLOW-UP ----------

QUESTION: Thanks Matt,
I have flushed the valves and didn't see any debris.  The flushing also didn't help with the water problem.  I would like your recommendation of a filter and any other suggestions you may have to fix the problem.

Answer
Matthew, sorry I haven't replied for a few days, I've been on vacation.
A simple inexpensive filter is a VU-FLOW, style "T" It is a screen type filter with a clear casing. On the bottom of the casing is a plastic ball valve you just open to flush the debris out. They have a 24 mesh available, which should be effective for you purpose. The higher mesh number the finer filtering. I wouldn't recommend going much over 40 mesh since may require very frequent flushing. Sort of defeating the purpose of an automatic irrigation system. I would use the 1-1/2" or 2" size and use bushings to adapt to your pipe size. The larger filter will require less flushing.
Install it someplace that you can open the flush valve without making a mess. You may need to dump 2-3 gallons for a flush. If you find yourself becoming a slave to the filter, with frequent flushing, these can be fitted with an automatic valve in place of the ball valve. Then it is wired to the last zone of the controller and programmed to run 1 minute after each irrigation cycle. Obviously, that valve would be a different style than the type your using now. It is made to handle dirty water, but they are more expensive too.
I know many of the home improvement stores handle VU-FLOW or a similar product. Goggle VU-FLOW, and you should find a supplier for the product. Stay away from the "replacement cartridge" style.
Good luck and if you need more help, feel free to ask.