Dr. Diff 11.5" front brakes showed up tonight

Finally got out to do a "bed-in" of the front brakes. Several hard 55 to 15mph slow downs. Very nice braking that improved with many cycles. Straight as an arrow. With hard pushing on the pedal, when nearly at 15 mph, the back drum brakes would just start to lock up on clean dry asphalt. Is anyone running a special proportioner ? I have the 10" x 1/34" rear shoes.