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