There are many videos related to hydraulic jacks at YouTube, but most of those are not really very helpful for a rebuild, even though their titles are enticing. A floor jack is a really a bottle jack laid over onto its side and installed inside a frame equipped with casters, a handle, and a lift arm. This video
shows how to remove the bottle jack unit from the frame. The floor jack in the video is my Fleet jack with a different name and paint color on it. Some jacks are the least bit different. You may also want to view this video
If you do not want to rebuild the bottle jack unit yourself, you can use either of these videos to remove the bottle jack unit from the frame of the jack and simply take it to a repair shop near you. The cost of a rebuild at a shop is said to be around $150 to $200 plus parts. The e-How article linked in step 2 contains a word of caution. If your jack is a cheap import, it may not be worth a rebuild. The machining is not always as good as on the older models made in the USA. Further, the correct parts kit may be impossible to get. You may find it more economical simply to buy a new jack. Classic jacks made in the USA are generally considered worth the expense and effort of a rebuild.
This is a another video that shows how to dismantle
a bottle jack unit. There are some good clues in the video for reassembly.
Videos sometimes disappear from the Internet. Although it may seem redundant, I will also document all steps in this Instructable with photos and text so that anyone using it in the future will have all needed information, even if some of the better videos disappear one day.