Thou I haven't worked on a HWH jack, I have recently rebuild my hydraulic rams on my HWH slides.
They are relatively easy to rebuild. One seal, and one dust cap, one oring.
If you can get the ram out, it unscrews and you can take the rod seal out and put a new one in. total cost for parts less than $10.
If you don't want to fix it yourself, but, you can get the cylinder out, you can take that to any hydraulic repair shop and they quoted me $250 to repair it.
Pretty pricy to me, for the cost of the parts.
I will say that I did have difficulty working on the first cylinder as I had never worked on one before. But once I learned how to do it, It only took me 15 minuets to repair the other 2.
If you decide to repair it on your own, don't hesitate to hit me up ( before you take it apart ) so I can give you a few pointers.