I searched for info on repairing the common Craftsman (China) floor jack, didn't find much in the How-To department.
Mine was leaking fluid when parked, so bad that in a day or two it would only be able to lift a few inches, which is worthless for picking up a Cruiser. I got tired of filling it with oil (and mopping up under it the next day) every time it was needed. Before I went down to Horrible Freight to buy another china junk jack, I decided to dismantle and see what I could see.
The hydraulic ram assembly was easily removed from the bottom of the frame. Then it was clamped upright in the vise. A 54mm hub socket and inpact gun removed the big hexnut from the top of the cylinder. Inside the hex is the o-ring that seals the piston. The o-ring had been improperly installed by the goatherder who built it, so it was twisted in it's groove.
The jack has worked fine for several years, but the twisted o-ring finally decided to tear where it was wrinkled in the groove.
A trip to the parts store with seal nut & ram piston in hand turned up a perfect replacement o-ring- 1-5/8 OD x 1/8 section. The scratches and rust spots were polished off the piston, then everything was cleaned & slicked up w/ vaseline. Assembly was reverse of removal, except I did not use the impact, just a long 1/2"ratchet.
The first few uses of the jack had really bad lifting performance, but now it seems to have bled out the air pockets and is working good as new. No leaking out the ram o-ring, and lift performance is strong from top to bottom.