As a complete amateur who has limited experience, I have restored about
four pneumatic instruments for my own use.  Before doing so, I took
advice from those with lots of experience.

As a result, I tend to agree with Bernard Brown that hot glue, known
sometimes as pearl glue, is quite simple and cheap to use, particularly
for the inexperienced amateur.  Any mistake in gluing, such as wrong
glue consistency or allowing the glue to get cold, is easy to rectify.

Old leather and cloth can be removed by the application of gentle
heat using an electric temperature-controlled iron (a small electric
travelling iron is an ideal workshop tool) or a hot-air hairdryer for
larger areas of cloth/leather.

In addition, the blade of an old table knife, preheated by means of the
hair drier or in the 'cool' flame of a propane torch can be inserted
between pallet board and cloth/leather, to assist the separation in
very confined spaces between, say, the closely stacked pallets of air
motors.

Hot glue is also easy to use when replacing the bellows material.
Even in a confined space, the hot blade of that old knife will 'iron
out' any inconsistencies to ensure an air-tight bond between pallet and
bellows.  If there is still a leakage, the hot blade can also be used
to introduce a smidgen of extra glue and then used to 'iron' the
bellows to the pallet.

A useful back up to pearl glue is the glue gel used by piano restorers.
This sort of glue is supplied in a small pot.  The pot is put into hot
water until the gel liquefies and then can be applied in the usual way.
It eventually sets as hard as conventional pearl glue.

Finally, on the advice of an experienced organ restorer, I have also
used archaeval PVA.  This is very much like conventional PVA but has
the advantage that the bond can be reversed much like hot glue.  With
a little heat or the use of a 'steam kettle' jet, the glue gives way.

A particular advantage of this type of PVA is that it will bond to
residual pearl glue.  It is therefore ideal for those awkward places
where hot glue poses a problem and where the old glue is difficult to
remove in its entirety.  I replaced the air motors of my player piano
using only archaeval PVA and it is still going strong years later.

I await to be shot down in flames.

Regards,

Paul Bellamy

 [ See, for example, http://www.library.kent.edu/~nbirk/standards.html
 [ PVA (polyvinyl acetate) is one of the recommended archival adhesives.
 [ -- Robbie