The mc is out of a b body or something like that came with a Hemi. It has a 1 1/4" bore and I bought it from Napa. Yes it is a drum rear with the disk front...non power.
It is bore and travel length of your master cylinder and push rod that has to work uniformly. Master cylinders used with disc/drum combination brake systems will use a large reservoir for the disc brake portion of the system and a small reservoir for the drum brake portion. I have a feeling your master cylinder isn't capable of putting enough brake fluid to the extra large one ton wheel cylinders.
I went though this while taking a hydro-boost system off a Chevrolet one ton. To ever get a brake pedal I had to use a master cylinder off a one ton van in the end. We tried several with the same bore but the piston in the master cylinder had to be able to put the correct amount of brake fluid to each wheel to ever get a pedal. We had no problem getting enough brake fluid to each wheel for bleeding but never enough to build a pedal without pumping until we found that one master cylinder.