I'll guess that you adjusted the adjusters correctly so that they are causing the shoes to lightly drag on the drums. If so, then yes, it is probably time for a rebuild or replacement of your master cylinder. Did you remove the master cylinder and empty it? If so, then did you bench bleed it before reinstalling?
You could have a friend push down on the brake pedal while you are looking into the master cylinders reservoir. If you see fluid trying to squirt up while the pedal is moving, then the seals are shot in it.
