One lawn sprinkler zone doesn't work

Two of the five heads pop about 1/4 inch, instead of the full 4 inches, and dribble a small amount of water. They happen to be the lowest ones, but the lawn's slope is really pretty gentle, maybe five degrees.
In the room containing the water meter and the main shutoff valve, I can hear the water running to the bad zone. I timed how long it takes for the water meter to move a certain amount. The 14 seconds the bad zone takes is in line with the 14-19 seconds the other four zones take. This rules out a partially opened zone valve and a blockage in the pipe run between the zone valve and the head nearest it. It does suggest a break in that run. What's puzzling is that the water would be absorbed in the heavy soil surrounding the break at the same rate that water would leave the five heads if there were no breaks.
The 3/4" pipes are about 8 inches below the surface. I've let the water run for over a half hour, but couldn't detect wet spots in the lawn. I don't know the routing of the pipes, so it's just guesswork where to look.
For what it's worth, last year was the first time (in five years) that I ever had the system winterized by hiring someone to blow the water out of the pipes. (I'm in New Jersey.)
Any suggestions for locating the break?
Thanks,
Ray
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You'd be surprised at how much water can disappear. It's coming out under pressure, and if the jet is _downwards_, it may be building its very own sink hole. If the jet was _upwards_, you'd probably have a geyser.
Before starting to rip anything up, try removing the heads themselves. The strainers may be plugged with grit. Them being the lowest ones is suggestive.
Are the slow heads adjacent to the valve, or the t'other end?
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Chris Lewis, Una confibula non set est
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Chris Lewis wrote:

No, they are at the most distant end.
Thanks,
Ray
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If you can plug or cap off the risers on the heads that *are* popping up, that *may* generate enough extra pressure to make the broken head more visible. Most likely it's a cracked riser, although sometimes the bodies themselves get a hairline crack. Real fun finding the culprit, even when you know the location of the heads. Not surprising if winterizing caused the problem -- I've had heads launched out the ground by yahoos with those big truck-mounted compressors.
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