Paul Morris questions the use of leather in vacuum pumps of reproducing
pianos.  Many quality piano and orchestrion builders used leather in
their pianos as an alternative to rubber cloth.  Clearly, the correct
grade must be used but the resulting bellows will far out-live the
cheaper rubber cloth.

Fairground organs regularly use leather on their feeders, and the
expense of replacing it today can be well in excess of the cost of
fitting a blower, hence our 'non purist' friends ruin the originality
of an organ by fitting the dreaded blower.  No organ builder would
dream of recovering a set of organ feeders in rubber cloth; it wouldn't
last five minutes.  I find it hard to believe that some player piano
technicians will recover a box pump in rubber cloth when leather was
previously used.

In pressure bellows, additional sealing will come from the use of card
stiffeners, resulting in only two small corner areas of the leather
being un-backed and potentially more leak prone.  There is nothing to
prevent you from using similar stiffeners on suction bellows if you are
worried about leakage, although if they didn't originally have them
they are obviously not required.

An Arburo of my acquaintance has four large feeders maintaining both
pressure and suction reservoirs and is still fitted with its original
valves and leather, and still blows off regularly whilst playing, even
after sixty years.

Stick to leather -- it's cheaper in the long run.

Best wishes, Nicholas Simons, England