Mother Mopar has long been noted for their one size fits all proportioning valves that almost always cause the A bodied cars to prematurely lock up the rear brakes under hard braking.
Gutting the proportioning valve section of the combination valve, and placing an adjustable one in the line going to the rear brakes, and taking the time to do the testing and adjustment needed to get the settings right after you install it is the usual time tested fix.