I am very grateful for the thoughtful replies to my inquiry.  This
is a puzzlement so far.

1. As to the 40" & 6" measurements, they were done with a water
manometer at a test point in the hose leading into the bass half
of the stack.  Since Duo-Art switch is turned off, both stack halves
should be controlled by the accompaniment side of the regulator.

Yes, the piano is very loud.  I over-tightened the spring on the
accompaniment regulator in an effort to defeat the problem, but the
vacuum just seems to drain out of the system in four chords.

2. As to valve travel, we followed Doug Bullock's instructions
precisely in doing this.  We used a micrometer, and all valves are
set to within that specification.  It took a lot of time.

The stacks were yielding 9.5 inches out while putting 10" in, by the
way, also measured with the water manometer.

3. "Does the Duo-Art piano continue to play softly after it's done
playing the series of big chords?"  No, it bounces right back in a half
a second or so.  I have tightened the spring on that finger, on the
bass side of the accompaniment side of the pneumatic, quite a bit.
That boosted the vacuum in "Duo-Art OFF" mode.

4. "Sounds like you have some friction somewhere along in the regulator
that is causing the trouble for you.  Could also be the knife valve on
the accompaniment side having some seating issues?"

Perhaps, although both knife valves were lapped on glass and graphited
before re-assembly.  They seem to be operating correctly.

But my conclusion right now, this being a very early (1915) Duo-Art
with very large pneumatics, is that it just doesn't have the lungs.
There is no regulator as in the grand pianos.  There is no way for the
system to recover when drained.  It plays very well on Duo-Art rolls,
albeit very loudly.  There is space under the governor to install some
reservoir bellows.  I can't see how they would hurt anything and they
could be easily removed later, leaving hardly a trace.

Unfortunately this piano is now three hours drive from my shop, so
I want to go prepared for anything when next I go.

And perhaps the pump just isn't tight enough?

Joseph Osborne
Carlisle, Pennsylvania, USA