If you are asking about hyraulic or pneumatic functions take a look at the "Fluid Power Basic" Ebook at www.hydraulicspneumatics.com There is a chapter that explains pressure controls except Relief Valves that covers their operation and gives some circuits. Releif Valves are covered in an earlier chapter
A second Ebook, "Fluid Power Circuits Explained" is being presented and has a chapter on Reducing Valve circuts but will not come up for a few months. It is a book of Fluid Power circuits and covers all basic components and circuits. I use it to teach an advanced Fluid Power class for anyone interested in designing circuits. It is all presented in schematic form so you must be familiar with symbols to get anything out of it. Symbols and valve function are covered in the basic book.