Before machining a piston down to take a leather seal, I would be tempted to drill and tap the end and fit a ptfe one from a Premier to see if that improves things. You would lose a little off the stroke but if the Bronze ring is that bad then the slightly shorter stroke shold not make much difference if the Premier seal works.
If the Premeir seal works, you can then either leave it in place or go for the leather washer. If there was no improvement you could still go for the leather seal (I do like leather seals) but I'd be thinking that maybe it was the cylinder that was out of plonk.