I have that 'shimmy' problem in my Duo-Art grand.  The pump bellows
were rebuilt but the valves were not, because they seemed okay.  The
piano plays very well.  It has only that unpleasant oscillation.  The
legs are okay.  The problem should be perhaps in some flap valve (inner
or outer), but it's hard work to dismantle everything!

I suggest an idea.  The problem may perhaps be solved by putting a
weight in the right place on the large pulley -- but where, exactly?
I would like to avoid a complete rebuilding of the pump, and I hope
to fix the problem with a trick, i.e., putting a lead weight somewhere
in the circumference of the pulley, as in balancing car wheels.

Can someone suggest a method to find exactly where to put the
counter-weight?  Please answer in the Digest.

Thanks,
Leonardo Forti
Rome