Practically all jacks are rebuildable. Some may have odd sized seals that are not carried in stock by local hydraulic shops because the number of o-rings available and u- cups is very large and it would be very expensive to carry all sizes. Most hydraulic shops don't want to repair low priced imports, because it is not cost effective for them, so they tell the owner that the jack is not repairable. They only want to repair expensive equipment so it is up to you to take the jack apart and remove the main seal at the end of the ram. You have to measure the inside diameter of the o-ring (best done by measuring the groove on the ram) and the thickness, or cross section, of the o-ring. This is assuming, of course, that the main seal in this case is an o-ring and not an u-cup.
Take that o-ring to a local hydraulic shop and hope that they have that size in stock. In the cases where a size is not available, a different shape or size seal can be used but this requires some machining of the ram.
There are o-rings in inch sizes (SAE) and in metric sizes. Many of the inch sizes can be used instead of the metric sizes because they are about the same size.
1/16" = 1.5mm
3/32 = 2.5mm
1/8" = 3.5mm
3/16" = 4.7mm
1/4" = 6.35mm
Common metric sizes come in:
1mm, 1.5mm, 1.6mm 1.9mm, 2mm, 2.4mm, 2.5mm, 3.0mm, 3.1mm, 3.5mm, 3.6mm, 3.65mm, 4.0mm, 5mm, 5.7mm, 8.4mm. Other 0-rings "standards" may have additional cross section sizes.
Below is a link to a site that shows the different metric "standards" or groups of sizes of existing, but not necessarilly available everywhere, O-rings. There are japanese standards, german standards, french standards , british standards, ISO standards, Dowty standard, etc. One that is not shown there is the italian standard, which is the most complete of all the standards as it carries a huge amount of sizes and cross sections. As you can see, no one is going to have in stock all these sizes.
http://www.prepol.com/my-ppe/o-ring-sizes
Also, you can contact Torin and see if they sell a repair kit for you jack.
http://www.torinjacks.com/CustomerServices.aspx
Also, aluminum jacks have side covers on the pump body that are sealed with big o-rings and held in place by the jacks side plates. This could also be a potential leak spot. By now, your jack is low on oil, so it does not lift as high as when it has a full oil level.