Save the jack.
Most probable cause is that the main seal, which should be an u-cup, is broken. Take the pump apart by removing the big tank nut, then pull out the main ram and check the seal at the bottom. Measure the ram itself for the inside diameter of the u-cup and the outside diameter of the back up washer for the outside diameter of the u-cup. Also measure the height of the seal. Be absolutely sure you remove every particle from the broken seal as these can later jam the valve balls if allowed to flow freely inside the pump. It is also a good idea to change all the o-rings in the pump at the same time as the u-cup; o-rings do not last forever!
Drain the oil from the oil fill hole in the big, round, tank body. Do not open any valve plugs in the pump body to add or remove the oil. Do not mess with these valves unless it is necessary.
To remove the pump you will need a solidly mounted bench vise and a pipe wrench with a 4' or 5' cheater bar (pipe) to remove the nut as these are usually very tight. Do not apply heat to remove. Make a mark on the nut so you can tighten the nut back to the same position when reassembling.
There is a tutorial in GJ about repairing jacks made by HIBALL. Below is a link to it. It will give you an idea of what's involved even if the parts are not the same and in the same place. You can also send a Private Message to Hiball to get the parts.
http://www.garagejournal.com/forum/showthread.php?t=51105
The jack in the link below is made by Shinn Fu and should be close to yours with respect to the parts it uses. This parts breakdown covers three different jacks and one of them was very similar to yours. I make reference to this parts breakdown because Shinn Fu bought what was the Lincoln company, or name, and some of the jacks it makes now are basically the same ones Lincoln built.
http://www.shinnfuamerica.com/Temp/832.pdf
