When I converted my FRST to rear discs with a bias valve, I used a Mk5 escort ABS master cylinder with only two outlets and ran a split front/rear system. The bias valve is in the line that runs to the rear.

I saw in a magazine once they joined the lines under the car and then fitted the bias valve. To my mind it was a bodge as they effectively did away with the two circuits meaning that in the event of a line hose bursting or line getting damaged you end up with no brakes at all. If you're gonna do it then do it properly and run new lines in, as it is the youngest ERST is coming up for 20 years old so probably in need of replacement hard lines anyway?