I went through this last season. I used Rig Pro, 14 Regatta Way, Portsmouth, RI 02871 401-683-6966 They rebuilt my Navtec backstay adjuster, all new seals, fluid, etc. The only thing they didn't replace is the slightly crazed pressure gauge. Parts $114, Labor $130, tax $7.98 Total $251.98 I dropped it off on a Monday and had it back on Wednesday afternoon. Works flawlessly. There was a few tiny nicks in the shaft but they polished it out, no charge. Highly recommended.
Heres a tip: Depending on the size of the hydraulic unit, get a PVC plumbing end cap that fits over the body of the unit. Then drill a hole just slightly larger than the shaft in the PVC end cap. Mine was 1/2" The end cap installs on the unit, with the shaft passing through it. It serves as a sun/weather shield for the seal. The seal on mine looks like new after a season and a half. RT