Okay, WHAT is going on with my brakes??!

For the past year or so I've had to put up with a "soggy" brake pedal. I have manual front disk brakes on my '70 Duster (yes they are the Kelsey-Hayes) with pads and shoes in good shape. After replacing a front rubber brake line and bleeding the brakes (which took almost a whole 1-liter bottle of brake fluid) on the front side I fixed as well as both the rears the pedal had firmed up nicely. However only a day or so later and the pedal was soft again; it will go down about two inches and then suddenly firm up. I also have to make sure I press the pedal with some quickness, as the pedal slowly sinks all the way to the floor if I gently put my foot on it. It seems like air is getting into the system from somewhere, as I kept getting bubbles when I was bleeding the brakes even though I was sure nothing was leaking. Could this be a master cylinder issue? If so where should I get a good one from, as my current one is a remanufactured MC (I think Cardone or whatever) I got two years ago from Checker.