In general it seems like a good quality jack to me, except for leaking.If there are no signs of oil leaking, I would try to pump it up to its highest position, without supporting any vehicle, and push down on it to see if it holds.
I'm guessing since it's been lying on its side for so long, maybe it needs a few pumps to self-prime itself.
It's actually a very good jack. I don't think there's anything close on the market to replace it. I didn't check to see if there's a hydraulic filler hole. I know some of these bottle jacks just need to be topped off sometimes.
I popped the rubber plug out of my 2004 jack to top it off because there was oil all over the storage area. The jack worked fine even having leaked, it was just messy. Now it is stowed in a plastic bag. I think I positioned it so that the plug is up. At least that was my plan, but the storage holder may not have allowed that. vic