Mule, Your picture looked like it was not easily feasible to replace one valve at a time. How old is the manifold? Will the other valves start to have problems after you fix the leaking one? Can you live with the leak? If possible build a gravel sump that the water can drain to and leach into the soil. Otherwise it would be easier to replace the whole manifold and configure it to be able to replace one valve at a time. You would have a new manifold with no problems that may surface for a good length of time. Depends on your system's age. You may try removing the diaphragm, check for debris, clean and reassemble. Here is a link to a "How to" for a diaphragm: http://www.ehow.com/how_6522537_replace-diaphragm-sprinkler-valve.html There also utube vids as well. JMHO. Aloha

well, i hired a company that tried just about everything, to no avail. I hired a second fella and he turned the sprinklers on, and of course, it didn't change zones. So immediately afterwards, he took the top off of the Fimco and it was still full of water. He immediately said, there's your problem. Know that this was a brand new Fimco, and because I thought it could be the stem from reading a bajillion other suggestions, another new Fimco was put on and it still worked exactly the same - no zone changing. Anyway, the new guy immediately said, well here's your problem. It took him 10 minutes to evaluate. Here's the specifics - I hadhave a 4 zone Fimco and I was using 2 (one for the front yard; one for the back); the remaining 2 were capped off. All he did was remove the caps on the 2 unused zones on the octopus. Turned it on, and whalla! Has worked perfectly ever since (been about 6 months now and still good). Apparently, the inside will suction down keeping it from leaking while in use; but once off (as it does for a few minutes between changing zones), it has an opportunity for the seal/suction to stop (but it doesn't because the water is still inside holding the suction down). When the water drains, this allows the suction to let go inside and it can then move to the next zone. I couldn't believe that quick alteration fixed my problem, being that the first "Einstein" said I had to chane the foot valve; the check valve, put in new piping (as long as I had to dig out for the foot valve); had electrical/current check; had the pump taken apart and entirely cleaned ($100 for the pump cleaning in and of itself) - and I had all these things done from reading all the experts online saying, "I guarantee you, it's the stem; I guarantee you it's the valve; I guarantee you that your pump needs cleaning has debris in it, etc. etc.") I would suggest, first go to the Fimco (or whatever brand you're using) and after it stops sprinkling and didn't change zones, remove the top and if it's still full of water, take the caps off the octopus for the unused zones. The water has to drain out somewhere in order for the suction inside the Fimco to break loose and allow it to move to the next zone. If you take the top off after sprinkling and it didn't change zones, and you find there isn't any water in there, then this obviously isn't your problem. But if it is full of water, you just may find that removing the caps from the unused zones on the octopus is all you need to do. Of course, I found out after having spent more than $500 with the first sprinkler company that obviously needs to go back to school, and he still had no idea when he walked away (I replaced him.) Thank God the second fella knew what he was doing.

Thanks for your reply, The heads rest a little below the grass and the grass around the heads makes a little "wall" which keeps water there. The heads are a half inch down below the water surface all the time. I started noticing it a few months ago, but we were in rainy season back then and I thought the rain water remained aound heads. Now, almost no rain, but I still see the water hanging around the heads. What I noticed was the heads that hold water are all connected to the same valve(other heads controlled by another valve are fine), so I suspect the valve isn't fully closed. Yesterday, I manually shut off the valve in question and I checked the heads today... No water.. so leaky valve should be the reason, I guess. The system works like normal when it turns on, so I'd guess it's electrically fine. I now think the replacing the valve might be a lot of work if I couldn't find the same brand/model because piping may need to be modified and it might be a good idea to open the valve to check the inside first. Is it difficult to open the valve and check ? I'll post the pic later and ask for your suggestion where to look into.

It is important that the backflow preventer be higher than any outlet to prevent siphoning. All irrigation systems have zone control valves downstream of the backflow preventer so I cannot see how installing a hand block valve would be a problem. If you are running a hose uphill from the valve you would need the backflow preventer to be higher than the highest point you want to irrigate.