Write up for the 929 master cylinder:
http://robrobinette.com/mastercylinder929.htm
The write up is about 10 years old, so the prices have prob. changed.
As I said before, there is a good chance that even with upgraded brakes you won't need this. I didn't on my silver track FD with upgraded brakes all around. If your pedal is soft, I'd highly encourage you to do a full bleed before trying this... it may not be necessary.
David, I do remember your brakes were soft at RA, but I also remember there was some kind of obstruction in the lines going to the rears that was preventing fluid from flowing to them. Did you ever figure that out? I'd start there if not.
If I remember correctly, you can buy a master cylinder rebuild kit for the stock FD master cylinder from the local parts stores as well.... AND it replaces the stock plastic plunger(which is a point of failure) with a metal one.