Thank you to all who responded to my plea for help.  Everyone's first
suggestion, including mine, was to check the knife to see that it was
seating properly.  It was.  When checking I noticed I was getting
vacuum from the exhaust hole to the crash valve.  I pulled the top
cover to check.  Never found anything, but when I recovered it,
everything was working as it should.  There might have been a small
bit of debris there that fell out and I did not see.

I bench tested the unit and everything worked perfect.  Nice independent
control of both Theme and Accompaniment from off to full on.  I put it
on the grand and the Theme bellows would partially collapse and I could
not get a really soft Theme setting.  Once again, no problem with
Accompaniment.  The Theme worked, but not to my liking.

I got to thinking that maybe this is a vacuum pressure problem.
My test rig is an old PPCo vacuum box pulling about 60" at best.  When
I tested the Duo-Art it pegged my vacuum gauge -- so north of 60" and
probably closer to 80".  I switched to "Soft" setting on the dynamics
control.  Everything seemed to play much nicer at about 50".

The last discussion I saw on MMDigest was back in 1990s, and that
was very technical about vacuum at each Accompaniment/Theme setting.
I never got an answer to "What should the raw vacuum be right out of
the pump?"

I do not see any vacuum adjust for the Duo-Art grand.  There is a bleed
valve in the expression box, and this is bleeding and working properly.
I can leave the setting on "Soft" and be happy, but this tells me that
something is wrong, with which I am not happy.  Once again I seek the
help of someone wiser.

Jim Quashnock