Silicone oil (not adhesive) is often used.  I have read about the pros
and cons of silicon oil in AMICA's Technicalities.  I have never been
satisfied with the sealing ability of any flap valve leather I have
found.  I suspect the nap is too coarse and stiff.  Not being a leather
expert I only write from my experience.

I cover new flap valve leather with the thinnest kid skin from Player
Piano Co.  The kid skin seals much better against the wood.  I use
plastic glue or PVC-E glue (flexible) to adhere the kid skin.  Just
a thin bead of glue around the edges and one or two thin beads down the
middle (the long way).  Be conservative on glue use as not to add to
the stiffness or distort the flap valves shape when the glue dries.

When tested for leaks (removing air and sealing openings) the
reservoirs of my rebuilt bottom bellows take 150 seconds on average to
open.  When I seal the openings of the pumps for testing, the flywheel
does not turn at all.  Sealing with kid skin gives me 60 extra seconds
on bottom bellows.  Extra effort sealing the wood, lapping the sealing
surface (using glass block and sand paper), as well as using airtight
cloth, also pay big dividends.

Bill Maguire