I am very interested in this topic as I am trying to work out what I need to do to upgrade the brake master cylinder for my new set of 8.4" disks.

I have been looking at this master cylinder from Minispares.

MASTER CYLINDER FOR 1980 ON PRE SERVO (YELLOW IDENTITY TAG)
GMC227
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This FRONT TO REAR master is the latest model dual master cylinders, which was introduced in 1985. One of the notable features is the failure switch on the resovoir cap. This master with yellow identification tab is now also the only recommended replacement for all models from 1980-88,except those with built in servo.This replaces all the front to rear square type reservoir cylinders GMC173/198 and the GMC192/195. It should also be noted that this master has metric unions which are available in pipe kits for Left BAU5655 AND Right hand drive cars is BAU5654.

I believe it has a different pressure output between the rear and the front and is different to the aussie split system which operates diagonally and is which supposed to be crap if one diagonal fails.

It also is advised to use the Mini Spares Centre MS72 in-line adjustable brake regulator valve between front and rear brakes.
And for the 8.4" disks to use the GWC1126 rear wheel cylinders as a start point.

Any educated ausmini comments greatly appreciated.

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

The rear proportining valve to my knowledge is the MS72. I bought mine reconditioned from PBR.

And with your rear wheel cylinders im pretty sure you'd replace then with new 5/8" ones. Again i think they are the same ones you have posted.

I will be running a 8.4" disc setup also, so the last picture i posted is a setup ive put together from the information gathered with the help of the guys on here.