Thanks Hiball for all of the info here, very informative. I decided to try to repair a free HF jack I got from a neighbor. It is a 1-1/2 ton quick pump aluminum racing jack, model 60569. The right side piston got stuck half way on him, and he left it that way for a few years. I removed the pump assembly, removed the left side piston, and was able to tap out the stuck piston pretty easily. I cleaned the cylinders and the pistons well, inspected the o-rings and they looked pretty good. There is a slight mark on the cylinder wall, probably where the piston stuck, but the piston moved easily past it. I also removed and cleaned the release valve. I put it all back together, put in new jack oil and tried it out. I did not mess at all with the ram piston, it moves smoothly. The results:
It didn't seem to ever fully purge the air, I went thru the purge cycle many times, but I decided to try it anyways. The first part of the stroke doesn't move the ram much at all. When the lift goes up, there is definitely mushiness to it. Also the ram will not go full extension. The unit will hold ~ 50 lbs overnight, I did not try anything heavier. Also, I heard hissing from the right side safety valve when releasing the lift back down. I removed both safety valves (after counting the rotations to closed), cleaned, inspected and reinstalled.
I am wondering if this is a valve issue, or could it be that slightly scarred cylinder? I know the unit is not worth alot, I'm more working on it out of curiosity. If I get it functional I will use it on outdoor equipment, maybe up to 250 lbs. I would appreciate any thoughts on what could be causing the problems.