I had this sort of problem and it was the rear wheel cylinders, that I had replaced.
The part number was correct but it was really a wheel cylinder for the earlier Drums.
here is a clip from Mopar Actions Rick Ehrenberg's Disc o tech.

>> In the mid-1970s, Chrysler eliminated the rear brake residual pressure valve from the master cylinder. Its function was to prevent any air from sneaking into the system via the rear wheel cylinder cups. In its place, Chrysler substituted "expanders" in the rear wheel cylinders. (Expanders are dish-shaped steel inserts that exert outward pressure on the cups.) If you should decide to use a master cylinder from a 1975-up car, be sure you have the expanders in your rear wheel cylinders. For at least 20 years they have been universal in all replacement wheel cylinders and rebuild kits.

Opps, I jumped ahead of your question. I see now you have rear discs. I don't have an answer.