Sprinkler system zone looses pressure

I have an underground sprinkler system with 6 active zones. I have had all kinds of issues over the years that I have fixed, but I now have one that has baffled me. Recently one zone decided to act up. Basically, when the zone becomes active via a controler, it runs fine for a few minutes, then looses 90% presure. I have replaced the value, fixed wiring, tested my well, checked for leaks etc. with no resolution.
I don't understand why initially it works great then goes flat. All other zones operate correctly except this one. I've tested it over and over yet it still goes flat?
My system is twenty years old and has worked very well over the years and hear in Florida we don't worry about pipes freezing etc. It's loosing pressure somehwere that I have yet to find.
Any ideas?
Thanks.
------------------------------------- SteveO
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First thing I'd do is manually activate the valve for that zone. IF it still does the same thing, then the problem is either between what connects that valve to the other common supply for the other zones or downstream from the valve. If there isn't water boiling out of the ground and it comes down to a bad pipe, easiest may by to just pull another pipe run.
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On 6/4/11 5:15 PM, SteveO59 wrote:

Is it possible that a supply hose is collapsing internally? I'm thinking of a hose that would have several plies.
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Perhaos something like a slit in a hose that initially is closed, but then due tio the pressure gradually opens up, and then recloses after pressure is shut off You don't say if the pressure drop is sudden or gradual, that would be a clue.
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On Sat, 04 Jun 2011 22:15:25 +0000, SteveO59

Have you tried removing the farthest sprinkler head, then activating the circuit, letting it "geyser", and see if it still attenuates? Could be junk in the pipes that takes a while to stack up against the screen at the sprinkler head.
--
croy

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On 6/5/2011 11:38 AM, croy wrote:

I was thinking something like that or switch the wiring with another valve and test. Regardless, take the valve apart and look at the diaphragm again. If everything is working at the valve, look down stream.
Jim
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You have a leak.
And you don't know how to spell.
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