brake issues, out of ideas...

Hi all, I have a '70 dart that is a daily driver. Brakes are manual 9 inch drums. I replaced all the brake hoses when I got the car and had been driving problem free ever since. Recently the brakes developed an unusual problem, while driving in the city occasionally the brake pedal would fade entirely as though I had lost the bleed. Pumping the brakes would bring the bleed back. New master cylinder installed, same problem. No detectable leaks. While bleeding the system during the m/c replacement, the pedal would stop about 2 inches from the floorboard. I'm thinking proportioning valve, I'd appreciate any ideas or insight.
thanks