Pete, a 3-bar linkage connects the Duo-Art accordion stack to the shaft
of the knife valve.  It imparts a very non-linear relationship between
the accordion movement and the regulator shaft movement, and the
adjustment has a very big effect.  The design was an attempt to achieve
a logarithmic relationship, so that each little dynamic step would
produce a consistent change in volume (like about 1.5 decibels).
Unfortunately, when the linkage is adjusted for the nicest-sounding
logarithmic response, the vacuum regulation is poorest, and big chords
will not play.

Try this: loosen the clamps at both ends of the linkage while you hold
the respective shafts from turning.  Move the linkage position and
tighten the clamps again.  Listen to your test roll as it checks the
piano through all the intensity steps.

I admit I'm a bit hazy about the linkage details -- at has been many
years since I experimented.  Craig Brougher, have you had experience
with this linkage adjustment?

-- Robbie Rhodes