I would suspect the master too, when you check to see if the pedal sinks, you just just "rest" your foot on the brake pedal, that's how you can tell. Apply too much pressure and it will work. I had the same 10" drums on all 4 corners on my car when I got it. Never had to pump the pedal. Should be hard as a rock, all the time. Another thing that no one checks is the backing plates for the brakes where the shoes ride on the 6 "pads". These will develop grooves over time and cause weird brake actuation.If everything else checks out, I would take a look at the backing plates too. Easy check without disassembly of the shoes/springs is to remove the drum(s) and try to slide the whole assembly fore and aft, it should "slide" fairly easily. An oddball problem could be that that brake light switch is too "tight" on the brake master pushrod. Meaning, the switch is preventing the master from fully releasing. If the master doesn't fully release, the compensating port in the master cylinder can't fill with fluid which could give you a low pedal.
