http://www.amazon.com/Industrial-Hydraulic-Technology-Bulletin-0221-B1/dp/B000YE15K6
The above link is for a Parker Industrial Hydraulic Technology bulletin. This will give you a full understanding of hydraulic systems.
I am concerned about some of the above comments. It is important to know PSI = working force not flow. Flow = speed. While speed = inertia it is not the proper way to achieve results in hydraulics.
I have not read anything about pressure relief valves in the systems for example.
Please understand I am not trying to criticize, come across as a know it all, or perceived as arrogant. I just want to help if I can.