Odd master cylinder/braking issues

Disc/drum or drum/drum? Double check your self adjusters. If incorrect or on the wrong side they could be self adjusting inwards, causing the cylinders to expand and take up more volume of fluid (causing the low level in the master), to the point where the shoes get so loose they could feel spongy when the brake is applied, and **** a little bit when released causing some dragging and your dust buildup. Just a thought.