You can only use the MS72 adjustable brake regulator/bias valve with a front/rear split system. Definitely NOT the diagonal split type (for identification see 'Brakes - Rear brake bias adjustment'). These should have been 'X-ed' at birth. Makes the car handle very interestingly when the system only operates the diagonally opposed brakes. Scary stuff. Almost as much fun as when the rears only work on a front/rear split set-up - particularly as the PDWA is built into the master cylinder, and it still has to go through the FAM7821 pressure regulator valve! Damn near no braking on the rear. Either way - split system brakes are grossly inefficient when only using half the system as they are designed to do when a failure in the other half occurs. Quite frightening.
To install the MS72 into a front/rear split system, it needs fitting into the rear feed line. Determining which this is can prove difficult if the system has already been tampered with or new replacement components fitted. Namely the master cylinder.
If the FAM7821 regulator is still fitted and connected up to the master cylinder - then it's easy. The feed-line that runs to the rear will be connected to the right-hand lower port (as viewed from under the bonnet) of the regulator. Simply trace the feed pipe from the adjacent upper port to the master - that will give you your feed port to the rear brakes irrespective of whether the master cylinder is an early or late type. Consequently the other one will be the front feed. Make a note of this some place safe.
If tampered with - either bits replaced/removed that aren't standard or are missing altogether - it's a little more difficult. True, you have a 50/50 chance of getting it right, but 'guess-ology' where the braking system is concerned is very irresponsible. Early-type front/rear split system master cylinders used the conventional Imperial threaded fittings. In these units, the lower/bottom port feeds the front brakes, the upper/top port therefore feeding the rear brakes. The later master cylinder (GMC227 - supplied as a replacement for the early types and requires a conversion pipe kit, sometimes identifiable by a yellow plastic band around the cylinder body) has metric threaded fittings with nuts of two different sizes - bottom one bigger than the top one. The conversion pipe kit is necessary because the piston functions (and therefore the port supply ones too) are reversed. The upper/top port feeds the front brakes, the lower/bottom one feeding the rear brakes.
Incidentally - something to watch out for - the two thread sizes are very similar. I have seen a number of cars where someone has managed to screw the imperial threaded unions into the metric ones. Apart from this being a potential disaster, the wrong pistons in the master cylinder are operating the wrong half of the braking circuit if the cross-threaded unions stay fluid-tight that long!
Having identified which port does what, scrap the FAM7821 if it hasn't already been junked. Connect the front brake feed pipe directly to the front brakes, connect the rear brake feed to the front of the MS72 (into the middle of what looks like a big nut), blank off one of the side ports and take a pipe out the other side port to the rear brakes. I generally mount the MS72 to the bulkhead stiffening triangle that braces against the inner wing - convenient for connections and adjustment.
Useful part numbers:
MS72 Adjustable brake pressure regulator valve