All of zone 2's 5 heads rise about 1/8", instead of 4", and water
dribbles out. The flow through the water meter is about 1/3 lower than
when the other zones are on, so at first I suspected a pinched/clogged line.

The odd thing is, that if any of the other four zones are on, zone 2's
heads still rise the same amount, and the throw on the active zone is
reduced because of pressure losses caused by zone 2 not being fully off.
So zone 2 is never fully on or fully off. This sort of rules out the
pinched/clogged line theory.

From the controller, I measured the DC resistance of all 5 solenoids;
all the same. I replaced the diaphram in the zone 2 control valve (globe
style); no change. The water passageway that the solenoid opens or
closes is not clogged. There is no leakage around any of the valves.

In the house, there is a master valve on the supply line that opens if
any of the zones are activated. I supplied 24 volts to it to keep it
open. Then I opened each zone's bleed valve, one at a time. Zone 2's
heads again rose just about 1/8". The other zone's heads rose the full
amount.

One odd thing: in most of the zones, once I shut the bleed valve, the
heads retracted after about 15 seconds, even if I didn't close the
master valve. (That maintains the full water pressure to the supply
manifold.) In one zone, however, the heads remained fully up even when I
shut its bleed valve. The only way to shut the flow was the close the
master valve by removing its voltage. (Maybe I didn't wait long enough.)

Question: Should closing the bleed valve while maintaining the supply
pressure cause the valve to eventually close?

After writing the above paragraph, I decided to measure the shutoff
times. I started with zone 4; it didn't shut off within 2 minutes, so I
assumed it never would until I shut the master valve. Then I tried each
of the other bleed valves. Now zone 4 always came on, along with the
other zone (which, naturally, had reduced throw). I tried waiting over
15 minutes between testing each zone, master valve shut, and I opened
the bleed valves to relieve pressure while waiting.

With two zones on, there isn't enough pressure to raise zone 2 heads at all.

So now the problem has expanded from zone 2 not rising enough, to zone 4
always on.

So confusing.

Thanks for your insights.

Ray