Over the years I have acquired a number of Webley mk3's, two of which I had as a lad in the late 1950's. It was then I replaced the leather piston washers on both and they were used for a number of years before being stored and since then have had little use. Both will still hold air in the standard compression test (by cocking, raising loading tap and releasing trigger while holding cocking lever), no air leaking either past the tap or the washer. After almost 60 years both rifles have the same power as they did then.
On the other hand the polyurethane washer I replaced on my Original model 5 pistol four or five years ago has disintegrated AGAIN!
In terms of longevity it seems leather beats polyurethane hands down.