my experience:
PowerGear. One of my jacks started leaking. According to PowerGear, that particular model (original...2001) was NOT "rebuildable"...altho I'm sure the parts needed are available if you want to research...and have the time. Unfortunately, I did not.
Powergear will NOT sell you a new unit. You must go thru an RV dealer or a registered shop. (An extra 100 bucks for a free phone call I could make?) Anyway...the replacement jack was PowerGear # 500384HR....which, according to their chart, IS rebuildable. $684. I replaced it myself. I had to borrow a 200psi air compressor, plus a Snap-on impact gun (my 125 psi unit and Harbor Freight unit were simply not up to the task)to do the job. Even then, I had to grind off one of the bolt heads. New super-hard hardware from Grainger and there it was. I still have the original...maybe i will get around to finding the parts.
((Edited to say:......) I cannot find a shop anywhere near me that is even remotely interested in any rebuild. All they do is supply hydraulic hoses and fittings. I am aware that if I DO find one, and they tear it down and find ANY defects in the chrome piston, it will still cost me bucks for the tear-down and I would have to buy a new jack anyway.
All this...for FWIW and good luck.