Hi all,  In response/addition to this subject I want to add some more
information.

Just adding foot pedals will not do the job as mentioned by Randolph
Herr.  This is due to the fact that the pump pressure is much higher
than any human can pump.  This is easily noted by observing that the
equalizer is always tightly shut on Duo-Arts with foot pump.  Also
the spill valve will add to the frustration of trying to pump this.
Furthermore, the idea to have foot pedals is to be able to add some
form of expression and fun into the system.

What Julian mentioned about the British Duo-Art is correct.  However,
American pianos also came as a Foot impelled/Electric combination.
Those were known as the export model.  I have worked on a lot of those
and I also have one in my lounge.

The US version achieved the same as the British, i.e., the theme is
controlled by the pumping and the accompaniment by its expression
regulator.  This is done by having a slide valve inside the expression
box which when open, will bypass the theme regulator: any theme
perforation will cause the appropriate stack half to be connected to
the full input pressure which is the foot pump pressure.

This is simply controlled by a pneumatic on the left of the expression
box which sits in series with another pneumatic that controls the spill
valve.  Both these only have vacuum when the system plays from the
electric pump.  So, when these pneumatics close, the slide in the
expression box is closed and the spill valve is open.  This is all done
with a simple one-way valve that is closed when foot operated.

One other addition to all this is an extra 1/4" accordion pneumatic in
the accompany side.  This closes when foot operated to make things a
little louder with less pumping.

There are no extra pouches in the expression box and no levers to set
from Electric to Manual, etc.  I think that any standard US expression
box can be modified to have the extra slider and pneumatics.  To find
room for the foot pump will be a whole different story though since the
pump in my instrument is a little smaller and the motor sits on top of
it.

Maybe this info will help someone out there.

Kind regards,

Bernt Damm
Cape Town