You can go to the case web site and purchase the cd or the scematics for the 921b, you can also buy the book, it costs $58.00 plus shipping. Case has all it's manuals copy writted so it is hard to find copies that are leagle to use.
If your having trouble with hydrolic pressure the most common cause is a cylinder bypassing, on occasion the stacked control valves go bad but not that often. Each valve has a set screw for pressure and a popit valve that can get dirt in it and cause the valve to bypass pressure. however the most common failure that I ghave seen is the sesna hydrolic pump going bad because the hydrolic fluid gets water in it and damages the pump. the pump is alumminum and the gears are steel and the thrust washers wear out. To find out for sure what is wrong test the pump output pressure with a pressure tester, this is assuming you have changed the filters for the system. Also if you use the wrong fluid here is a book on e-bay that is $40.00 dollars for the manual.