The AAR did get the same brake parts as other disc brake ebodies. That being said, the correct master cylinder for 1970 was one year only and E body only, and hasn't been available from aftermarket sources since the late 70s or early 80's.
If you have your original MC, which is identified by having a 2229171 casting number on the bottom, it is a rare and valuable piece. I have used a place in Minneapolis to restore master cylinders including several '9171's. They resleeve the bore with stainless steel and know what they are doing:
http://brakeplace.com/I'm not sure of what they are charging now, but it is probably in the $175 range. If you use them, tell them Brad from Illinois says hello.
Whatever you do, don't use White Post Restorations for these master cylinders, they use a similar but incorrect kit and when you install it on the car, it doesn't work!!!!!!! Has happened to a number of people so don't make the same mistake!
There is a section on this board for brakes, which would be the correct place for this question.