Lawn sprinkler popup heads

Two of my Hunter PGP 4" sprinkler heads seemed to be working okay based on the throw of the water. But I noticed both had pools of water around their bodies, so I assumed there was a leak, perhaps around the cap, and replaced them.
At one of replaced heads, there was no longer a pool around the body or in the still-open hole. But the other one leaked a little while the zone valve was on, but continued to leak much more when the zone valve was off!
There are five heads on this zone. Due to the slope of the lawn, the leaky head head is downhill from the three that are closest to the manifold valve, so I figured that maybe gravity was a factor, not that it really should be. The leaking continued for at least 30 seconds after the zone valve was off.
One more head, a different model with a fixed 1/4 circle pattern, is at the bottom of the run. No pooling there.
Is my leaky, new head defective?
Thanks.
R1
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Rebel1 wrote:

I have a 7-zone system that was installed in (I think) the late 90's and it has these Hunter PGP springlers.
http://www.sprinkler.com/skin/frontend/sprinkler/default/images/media/pgpcrosscut.jpg
http://www.mapletrade.co.nz/Images/Irrigation/pgp.jpg
Two sizes, but mostly large (not 4" diameter, but maybe 4" pop-up). I've been replacing many of them over the past few years - primarily because they get stuck at one end of their rotation pattern and just sit there and click unless I give them a tweak by hand and then they'll make one sweep and come back to the same position and sit there.
Has anyone ever taken these apart to see if any gears can be cleaned or fixed?
The other reason I've replaced some of them because they seem to leak around the joint where the sprinker retaining collar gets screwed into the base canister. I can't figure out why it leaks (it seems clean) but I also notice that new units (like these old ones) don't seem to have a gasket where the collar meets the canister.

Well, yes. These are self-draining sprinklers. I think when the pressure is turned off and they re-seat themselves back down into their canisters (because of the big spring they have) it must open or expose some passage where water can passively drain from them.

Unless you cracked the elbow or the extension where the canister is screwed into, I'd say no.
Be aware that the extension tubes are very soft plastic (at least the black plastic multi-segmented ones sold by home despot).
I was done messing around with these things about a month ago. Some still need tweaking, but I'll do that next summer.
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Thanks for the feedback, Home Guy and Bob. Tomorrow, I'll be better educated when I go back to the place I bought the heads, a professional sprinkler company, not HD or Lowes, and describe the problem.
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What problem? It's normal for popup heads to leak slighltly around the seal between the popup part and the base when it's watering. They tend to leak more with age. You just don't want a lot of water spewing out. It's also perfectly normal for water to come out of the head for 30 secs after it's shut off, if it's a downhill run. Why would you care about water trickling for 30 secs into the lawn?
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I've taken mine apart to clean them and I don't see anything in them that would "turn off" the water at the head. I would expect a low one to drain the from the higher ones.
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