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Check this before you get the proportioning valve. Remove the rear brake line from the master cylinder. There should be a metal seat inside the master cylinder where the brake line attaches, remove this with the proper size screw (self-tapping sheet metal screw works good). Remove the check ball behind this seat and reinsert the seat. On a drum system, this keeps fluid from coming back into the master cylinder, don't want that with disc brakes. Try it with the stock proportioning valve, many do okay with it. If not, get the adjustible one with the knob on it.
This trick works when converting old CJ's and Commando's to disc on the front, so we'll see if it works on a newer vehicle on the back. [addsig]