I can't tell you of any specific site to learn about hydraulics but after you learn enough to want specific questions answered you are on a pretty good site right here. Before you start your Internet search, check your spelling of the word hydraulics, I misspell it all the time. There are many years of knowledge on the TBN associated with hydraulics and many other related issues. Search Ebay for books on hydraulics. If you don't find it the first try, keep up the Ebay searches and eventually you will find what you are looking for.
Good luck with your search for knowledge and welcome to TBN.
Farwell