This is an interesting and complex subject!  The key to understanding
the Duo-Art's knife valve regulator and stack leakage is that the Duo-
Art regulator relies on two balances which define its steady-state
operation:

1)  Air flowing into the stack (via leakage plus valves and bleeds when
notes are played) must be balanced by air being extracted from the
stack via the knife valve.  Therefore if more air flows in, the size of
restriction caused by the knife valve opening must become larger.

2)  The physical geometry of the regulator means that the regulating
spring is stretched further as the knife valve opening closes.  Because
the force from air suction inside the regulator must balance the spring
tension outside in the steady state, it follows that stretching the
spring further increases stack suction.

It is the pneumatic balance point where the air flows in and out are
equal that defines the amount of stretch in the spring, and therefore
sets the suction level applied to the stack for a particular position
of the knife valve (as defined by the accordion pneumatics).  The
system has an innate 'droop' in suction level as more air flows in
(such as when notes are played).

So, there are two obvious failure modes in the pneumatic balance:

a)  If the knife valve does not seat properly, it will extract more air
than is coming into the system, so the regulator will slowly suck
entirely shut, stretching the expression spring to its limit and
applying maximum suction to the stack.

b)  If the air coming into the system is too great, the knife valve
must open too far, thereby reducing the stack tension and making all
playing too quiet.

So, to get a Duo-Art to work adequately the knife valve leakage must be
significantly less than the air leaking into the stack.  This tends to
be the case, because 88 valves will inevitably seep a tiny amount.

Excess air inflow is less catastrophic but gradually overwhelms the
expression system.  The characteristic leak-vs.-suction of calfskin
valves will be factored into the expression coding, but leaks from
other sources such as huge holes certainly won't be.

As Robbie suggested in his footnote yesterday, there are many failure
modes for the Duo-Art, and it might be instructive to try and list
them!

Julian Dyer

 [ Where should we begin?  ;)  -- Robbie