The single most important aspect of purchasing an aftermarket master cylinder (or any non-original master cylinder, for that matter) is sizing. Make sure you are getting one that is the correct diameter. Get it too big and your pedal will be rock hard. Get it too small and it'll be too soft. There is a lengthy write-up on Hot Rod Heaven's website by Dean Oshiro. It is a little long but the last section, in particular, deals with master cylinder sizing. It's well worth the read...

http://www.hotrodheaven.com/tech/brakes/brakes1_index.htm

As most of you know, I've done numerous upgrades to the brakes on my '67 122 including Wilwood 4-piston calipers, 11 3/4" rotors and a Wilwood master cylnder. I was fortunate enough to have a good buddy that had read the Oshiro article so we plugged a bunch of numbers into his formulae and came up with the size we needed. My brakes are nothing short of amazing. I'm doing it with a single circuit system...

Dale