Following on from Roger Waring's idea of only sucking as much air as is
actually required in a player piano may I pose my own challenge to the
readership.  This is a problem I considered some years ago but I never
built the resulting design.

Problem: In a hand-cranked street organ, obscenely called 'monkey
organs' in USA, one is providing a constant flow rate of air all the
time.  The feeders are designed to supply the maximum potentially
demanded airflow at all times, although some very short high demands
may be able to be catered for by the reservoir, but in a portable
machine the size of the reservoir is limited.  The paradox is that when
the music is at its quietest the spill is at its loudest, but more
importantly for the operator, he is wasting all that muscle power.

The challenge is to design a simple mechanical linkage, within the
confines of the existing organ case, that varies the stroke of the
feeders in accordance with the demands of the music.  The result being
that the crank torque will also vary, and there will be no wasted
effort.  If correctly designed, there will be no need for a spill
valve.  It is not really worth patenting the result as no one will make
their fortune from it, but it would be a novelty if such a mechanism
were available commercially.

Have fun, Nicholas Simons, GB