Hello and welcome.
Lets back off from your information/question dump and take things one step at a time.
The brake master cylinder is located in a most "
convenient" spot below the driver's feet. There is a trap door on the floor that you'll open up. You'll then see the top of the master cylinder, with possibly a hard copper line (vent line) coming out of the top. Take off the vent line, then take off the master cylinder cap. This is where you fill it. Military specification is to use DOT 5 (silicon) brake fluid. Don't mix it with DOT 3. DO NOT! Some folks do a complete alcohol flush of the system and change fluids. I recommend knowing for sure what you have in there before adding to it.
Also, like everyone else, I highly recommend adding a remote reservoir for easier fluid level monitoring. Something like the following:
http://www.bigmikesmotorpool.com/pro...-reservoir-kit
See the massive list of Deuce Technical Manuals (TMs):
http://www.steelsoldiers.com/showthread.php?77607-TM-s-for-the-Deuce
I opened TM9-2320-209-34-2-1 Maintenance Direct Support and General Support M35 Multifuel 1 of 3, and on Chapter 12 section III was a lot of diagrams of the master cylinder and air pack pulled apart.
Also, just searching this forum. See the search field in the top right below "logout"? I typed "deuce air pack rebuild". First result was this amazing thread:
http://www.steelsoldiers.com/showthread.php?82753-MY-Airpack-Rebuild-with-Photos!
Sorry, I don't know about diagnosing the master cylinder. Try your hand at searching for it.
Post your progress and rebuild/fixes in the Deuce forum. With lots of pictures. We love pictures.