My Air Force career field involves rebuilding 5, 10, 30, and 60 ton jacks for jacking aircraft, as well as maintenance stands that jack up hydraulically. Typically we remove and replace cylinders, but there's been quite a few times we've had a lack of parts unavailability, or a hydraulic cylinder was priced something horribly stupid, so I've had my grubby fingers inside quite a few. Last time I deployed I did a lot of teaching with airmen whose jacks wouldn't pass a load test. The 30 ton tripod jacks are pretty much a cylinder with outriggers, so when the hydraulics go bad your only option is to dig in and repair them. Very frustrating. Be tediously clean and careful.
The B1, B2, and B6 maintenance stands all use the same 8' long hydraulic cylinder, and about fifteen years ago at home station we went to replace one that was puking fluid. Supply listed the cylinder's price at something stupid like $5,000. I looked down the empty bore and it had a bit of corrosion. The next day I brought my engine bar hone. I don't remember what combination of adapters we used to hook it up, and I think there was some duct tape involved, but I honed the bore out, put new seals on the cylinder, and put it back in service.
If your jack's bore isn't clean, consider trying a wheel cylinder hone before you trash it.