So just overhauled this quickly, as it's new to me...
- the breach seal was toast, so I made a new seal with a couple of hole punches, a big hammer, and an old leather belt. Job done, perfect thickness
- piston head was also toast, so I made a nylon o-ringed head for it. There's still over 1/2" of clearance from the cocking link, so I will probably replace this with a diana 28mm parachute seal, which will gain another 1/4" of stroke, for more power and a softer cycle Or even a BSA standard type leather seal.
- the spring arrangement (1 skinny and 2 short/fat) was just silly, so I found a suitable skinny spring and did away with the lot. This new spring was a medium wind, 3.0mm wire, and gave about 1.5" of preload. Cocking effort was moderate, but not silly low, so seemed about right.
- just did a general moly relube in all the obvious places, and re-assembled
After a dozen or so shots, it was behaving quite nicely. Fast firing cycle, just a fraction harsh, but nothing unpleasant. Just for curiosity I checked the power - 5.5 FP on low power, and 7.5 on high. I'd expect that to come down a smite as there's a little combustion, but if I want more power I'll change to the seal as described above.
Overall very pleased, and impressed with this baby.
Only big decision I have left is whether to refinish it or not - it's on that line between whether it's too tatty as is or will it be devalued by a refinish... most likely I'll leave it as is and just enjoy it. Nothing like renovating and enjoying shooting a 110+ year old gun