I went through this about a year back, instead of screwing around with it I just bought a new Arcan 3.5 ton jack from Costco for ~ $120. For me it was worth it for the saved time and aggravation of pulling it apart (mine had popped a seal) and finding the seals or a shop to deal with it.
Arcan Quick Lift Professional Service Jack - 3 1/2-Ton, Model# XL35 : Amazon.com : Automotive