Hi all,  I'm finishing a Duo-Art system and am now working on the pump.
I cut new flap valves from a piece of heavy leather that works for
almost all of the pieces, but there are two flaps that I installed that
simply don't seal tightly.  The leather curls slightly out.  I verified
that this was the problem by putting in the old flap-valve leather:
immediately it was as tight as possible.

I can easily use the old leather to solve this problem (it seems to
be in great shape after 80 years sitting in the piano) but I would in
general like to know if there is a better solution -- a way to make the
leather lie flat and get a good seal.

The knap of the leather is short and even and seals well when I tried
a simple experiment of taping it across the leather (to make it lie
flat).  Obviously this is not a solution, but has anyone a better one
that is tested and convinces the leather to change its position?

I'm obviously worried that if I somehow change it now that it may
decide to revert to its original shape when it relaxes over time or due
to temperature change, humidity, etc.

Best regards,
Eliyahu Shahar