Hi,  In response to Eliyahu Shahar's posting, I too incurred this
problem when renewing the flap valves on Duo-Art pumps and indeed
several other types as well.  My solution was to glue the new flap
valve leather to an identical sized piece of bellows cloth (stock
no. 55 from PPCo).  This effectively stopped the leather curling up
at the edges.  Because of the extra weight I also would put a couple
of wire bridges over the flap to prevent it moving too far away from
the valve surface when in operation.

No, it's not original, but this is one of those logical improvements
I sometimes mention.  It works very well on many different applications.
The increase in thickness is not usually a problem on the external
flaps and very rarely on the internal ones.  Good luck.

Paul Camps