Maybe I missed something but I don't recall the car you are working on....In addition, there are a few mistakes I see here.
NO Diplomat came with manual brakes. The master cylinder from a Diplomat is sized for power assist.
Any disc/drum system needs an actual proportioning valve. The front calipers use a much greater volume of fluid as compared to the rear. The stock Mopar disc/drum proportioning valve held off pressure to the rear until the FRONT built up pressure. By using what is known as a "distribution block", (No metering, just a junction for the brake lines) your calipers are only getting the volume and pressure that the old front wheel cylinders received. NOT good and nowhere near enough.
I have a bunch of actual disc/drum proportioning valves that would work. The ones from mid 70s A body cars are pretty easy to find and work well. PM me if you want a screaming deal on a used Prop valve.