Chris Lewis wrote:

Orbit brand parts (especially valves)?

Don't know the brand. I don't see it stamped anywhere. It's a
discontinued model, but replacement diaphrams are still available.

Does zone 2's valve affect the flow rate at all? Or is it always
the same dribble, regardless of bleeder valve/control setting?

Always a dribble, whether opened by bleeder or solenoid.

I somehow got zone 4 to behave, so the only problem is again just with
zone 2's head not fully popping. Now, zone 2 only dribbles when it's
selected; it no longer dribbles when the other zones are selected.

These valves can sometimes exhibit odd behaviour. Unscrew the
solenoids and make sure that the orifices aren't scarred up.
In certain vintages of valve, the orifice can "slip" out of
optimum alignment with the solenoid, and exhibit a variety
of behaviours:

- won't turn off
It does turn off.
- bleeder does nothing
Bleeder works; causes dribble, just like when solenoid activates.
- bleeder arm jammed and cannot be moved
Don't have this problem.
- won't turn on at all.
Don't have this problem.

Try activating the solenoids after they're unscrewed and see if
it actually moves.

Before my first post, I had swapped solenoids; same symptoms. The
problem is either in the #2 valve or the pipe from its output to the
first head.

Since the flow, as timed at the water meter, is lower for zone 2, I
suspect a pinched pipe, although there are no nearby trees. Perhaps
there can be a break such that the loss of water through the break is
great enough to keep the heads from popping (and releasing a lot of
water), but low enough so the flow through the meter is low. I have to
think about that. The soil isn't clay, but is slow draining.

Thanks for the input.

Ray