I have a 1990 Chevy with a Detroit 8.2. I also have hydraulic brakes. These chassis came with some different brake systems. My bus has the Bendix system. Check out link to the PDF file and see if that is the system you have. You can also call a GM dealer to look up the VIN and find out what system you have.
You will find everything about the sytem in the link to the PDF file i attached. If you don't know how old the fluid in your system is do yourself a favor and replace the fluid before your master cylinder gets pitted and the seals start leaking. That cost me some money and time that I did not have to spare.
http://www.bendix.com/bendix/downloads/ ... 206602.pdf