ORIGINAL: The_Shark
if you run the pads/shoes too long (or if you have a small leak) you may run the master cylinder low
Master's full.
if you hit the brakes when its low you can make it gulp air, enough air and it will make your pedal like your stepping on a marshmallow, filling up the master will not fix the problem, fill it up, bleed it and see if that works.
Won't fix it. The long and short of it is: The entire system is FUBAR. We think a previous owner might have done a lot of brakestands, as the rear drums are positively cooked while the front discs aren't. It's had some sort of jerry-rigged crap done to the rear lines too, and the rear lines are leaking at the slave cylinders as a result. On top of that the brake booster is torn, so that doesn't work.
So we're just going to replace everything from the pedal out.