My indexer cylinder on my Javelin started leaking past the seal today (leaking back into the valve when it's at the far extreme).
I ordered a new one from Bimba, but when I was looking through their catalog, I found that they have rebuild kits and parts.
I'm considering ordering up parts from them to rebuild it, so at least I have a spare...
I know that the rest of them (other than the table-up cylinders) aren't rebuildable, and I do keep spares of them on-hand, but for the large ones, it might make $$$ sense to not replace.