Working on a Mini Cooper S. It has a master cylinder that is really stubby, looks like this:
It had the usual split diagonal brake system with 6mm lines to the ABS unit and 4.75mm lines out to the wheels. Brake piston diameters were 48mm front and 34mm rear.
I say "were" because the car now is getting calipers with "54mm" (2 1/8") pistons in the front and "38mm" (1 1/2") in the rear. That is one thing. The other thing is that, to make life and plumbing a lot easier for the hydraulic handbrake, the system will be changed to a front/rear split. Front brakes getting 1/4" line to a tee and then 3/16" out to the calipers, rears will be 3/16" all the way back since I know from experience that works just fine.
What I don't know from experience is how well one of these little stubby master cylinders works stroke-wise once one circuit is no longer feeding a 48mm and a 34mm caliper and is instead feeding two 54mm calipers. I can't see it running out of travel, but I also have never used an "even volume" master on a front/rear split before, and if I am going to change anything now is the time.
I suppose it could also just be a case of way overthinking things...
I'd imagine (and I'm interested to see Angry's input) that the current master has equal output on each circuit which would/should now give you more rear bias, requiring a proportioning valve on that circuit. Unless there is a stock proportioning valve still in the system that I'd imagine could work for you.
No proportioning valve (initially) since Subaru, the brake donor, tend to be excessively forward biased. I also just remove them on my Mazdas and the brakes work better because Mazda was excessively forward biased.
More trivia. The Mini master cylinder is .875" bore, the Subaru had a 1" bore (!), which was the smaller of the two options (!!). This is really curious because Subaru have generally always had really soft, mushy brake feel, I didn't realize they used master cylinders that large. They must make it up with brake assist. I have the brake pedal from the Subaru in the parts pile, now I am really, really curious what the pedal ratios are.
In reply to Pete. (l33t FS) :
The mush in the Subaru pedal feel could be due to other things like flex in the caliper assemblies, brake soft lines, etc.
Flex in the booster itself...
...BMW E90s are front/rear split and they use the same master cylinder. With different bore sizes, in case I need to play with that.
Moving on...
Simplified calculation to compare volumes:
Old diagonal circuit 1: ((48)^2 )+((34)^2) = 3460
Old diagonal circuit 2 = same
New front circuit = ((54)^2)+((54)^2) = 5832
New = (5832/3460) = 1.68x old
My experience: we generally select master cylinders with about 2x the circuit volumes, to allow for variations in friction vs temp (ie fade), humidity, etc.
My opinion: your proposed new arrangement only has 15% volume reserve for output variation on the front circuit. That assumes the Mini MC followed the 2x rule. I would not run that on the street. I also wouldn't run that on a track car. It might be OK for autocross.
Options: find a car that was front/rear split with 54mm front calipers, and check the bore and stroke of that MC, and use that info to guide your choice.
I think I see what you are getting at, thanks. Very useful info.
The E46 (not E90, my bad) 325i uses this master cylinder with a .937 bore (15/16" just like God and Girling intended) and 54mm front pistons, 40mm rear.
Since there is a bolt in way forward if this does not work, I'll go with what I got for now and if the pedal is crap, I'll worry about it then. Application is not for street, track, or autocross, and I fully expect the power brake booster to be depleted on a regular basis so having some additional hydraulic leverage may not be unwelcome.
This is one of the few times that a BOLT IN solution actually exists for this car 
Just because I said I would.
The Mini brake pedal has a truly atrocious 3.5:1 pedal ratio (a tad under 3" to pushrod, roughly 10.5" to middle of pedal).
The Subaru brake pedal, on the other hand, has a merely icky 3.9ish:1 pedal ratio. 2.5" to a little under 10".
I drilled out my '81's pedal for a 7:1 ratio in anticipation of converting to manual brakes... stock is 4.5:1 and it sucks when the booster is empty.
