Actually, they CAN be rebuilt. Looked into this when this same question came up over a year ago and it peaked my interest.! Why not? My Biesemeyer leaked a little too, so there had to be a cheap fix right?   

Heres the problem.......They leak because possibly the packing is worn out, a gland seal has failed or the shaft off the piston has become corroded, bent, and/or pitted.......maybe .too much pressure........any way you look at it, not good.

Cheapest tiny hydraulic cylinder repacking kit I came across (Used all my old sources from way back when I rebuilt my pops Backhoe after the barn fire........another story) was $65. They had no problem matching me up with what was needed to do what I wanted to do, but the secondary problem was replacing and polishing the hard chrome on the piston/shaft assembly, which, more than likely, was the culprit of the failure........2 minutes into that phone conversation showed me that this was a dumb conversation to be having to begin with, more on MY part than theirs........

$119 was the cheapest I found that cylinder for. You'll NEVER be able to rebuild it for less than that, really.......   It's disposable, it is what it is.........

Ray