If you want to try Hydrophane on your leathers, do a Google search and
you can find an uncounted but large number of equine shops that sell
the stuff in varying metric quantities.  I bought a two-liter jug a few
months back just to try out on several original leather surfaces in a
1952 Mulliner Silver Wraith Rolls-Royce.  The car had been in storage
since 1967 in barn in upstate New York.  The leather was dry like paper.
Several treatments with Hydrophane brought the leather back to a supple
state.  I then used a coating of Connolly Hide Food to finish and seal
the seats.  They look great and we'll see how they wear.

A bit off topic I must confess.  The only pneumatics on this vehicle
relate to automatic self lubrication of the car's grease fittings.
I must confess that I have not tried these materials on mechanical
musical instrument leathers of any kind.  It might be worth a try but
I don't have anything to experiment with at this time.  I doubt that
this would be a general solution to leather problems.  It's worth a try
if you're about to restore or rebuild anyway.

Bill Finch