Question
Matt,
    Thanks in advanced for your expertise because we are absolutely stumped!  We have a Rainbird sprinkler system.  One zone was leaking out heads constantly.  Replaced the valve worked great.  Then after a couple of weeks started leaked again.  Had valve totally replaced again by professional.  We can now hear the valve turn off, doesn't sound like there is any water getting through, no leaking out heads.  However, the lawn directly around the area of the last two heads stays sopping wet, even after water hasn't been run for a couple of days. It drains out the side yard and makes a visible pond in the dirt that stays constantly.
    If we turn the water valve off to the entire sprinkler system - it dries up until we turn it back on.  If we turn the control panel in the garage to off it also dries up until we turn the panel back on.  
    Have had it looked at - everyone swears the valve is fine.  It is the low end of the system so we would expect some drainage... but not for days after it last ran.  
    Sure hoping you can give us a clue where to go next!
Thanks Much, Kris

Answer
That's a tough one.
I would examine the valve carefully. You may have to dig it up, again. Look for any signs of dripping around the top or sides of the valve. This could allow the valve to weep.
My guess is the same reason the first one went bad is the reason for #2 and #3. There is probably a tiny bit of debris and it is causing the valve to not completely close. You mentioned it is the lowest valve. That would be where the debris would accumulate.
My best suggestion would be to dig it up. Remove the valve and connect a short piece of pipe to the fitting. Then turn on the water and flush, flush, flush. It would be a good idea to run the first bit of water into a bucket. This way you can see what type of debris your getting.
Good luck, reply back if you need any help.

Matt