I had one Power Gear jack rebuilt this year. I removed it and took it to an hydraulic repair shop. I could have done it myself, but I couldn't open it up to access the seals inside. They charged me $200 Cad. and it was ready in two days. I tend to do all my mechanical maintenance, but in this case, it was probably a good idea to pay someone that knows what they are doing. I will do the same next time.
The shop also told me that in some model of jacks they are very difficult to open them up, but they have found a way to do them also. I don't know where you are located, so I won't mention the location of this shop. I am in St. Catharines, Ontario, Canada.