I have 3 Rainbird solenoid sprinkler valves covering three zones in the back yard. Only one of the three valves leaks from the anti siphon portion, after that zone finishes watering. It doesn't leak while it's watering, only once the cycle is finished. Then a bunch of water comes gushing out. I tried replacing the cap with a new one, thinking maybe its the O-ring or something, but it still leaks out a heck of a lot of water. Almost as if it's draining out all of the water in the line. What could be causing that to happen? How can I fix it without having to replace the entire mechanism? I've got a huge puddle of water out there being wasted.

Is the valve installed higher than the sprinkler heads? Did this just start happening or has it always been like this?

It's been like that for awhile now but i don't believe it did that before, but honestly not sure. And yes, some of the sprinklers in the backyard are on a slope and therefore higher than the valve.

It sounds like it's doing its job then. There's more discussion at [www.plbg.com]

ANY water in the pipe higher than the valve WILL leak out of those anti-backflow valves.

Don't forget that the sprinkler heads themselves also have built in drains. When the zone shuts off, if the pipe runs downhill from the solenoid to the heads, the line will drain out of the heads and you won't see any water at the solenoid. If the pipe runs uphill to the heads, the water will want to flow back toward the solenoid when the zone shuts off, and the anti-siphon or backflow preventer on the solenoid will allow the line to drain there to prevent any irrigation water from re-entering the house water system.

This is a not a waste of water but a safety measure. Irrigation water may have been exposed to fertilizers and other chemicals, and this prevents irrigation water from entering the house potable water system. Solenoids are designed now to drain off that water when the valve closes.

Consider maybe digging a hole 6" deep or so by the solenoid valve and adding some rocks or gravel so that the backflow can drain rather than puddling.