Pinto Car Club of America

All of a sudden my brake pedal hit the floor, brake light on, barely any braking ability. Luckily happened on a side street, moving slowly! 
I was expecting to find a leak and the front channel of the master cylinder empty but found neither.  Full master on both channels and no leaks anywhere. 
Bled the lines. Well... as best I could. The bleed valve on one of the rear drums was rusty and the dang thing broke off while trying to open it. Didn't want to take my chances with the other. So, rears aren't bled but I was skeptical that would help, given how badly the brakes failed.  Seems like the rears are all that's working.
You know one odd thing is that there seems to be pressure in the pedal if the car sits a while but then when I start it up and try to drive, the pedal loses pressure. Seems strange for a manual brake setup. 
Anyway, I'm at the limits of my understanding on how the brake system works.  Any help would be appreciated.

master cyl croaked  :P  try to find new, not rebuilt

My friend's jeep is doing something similar, pedal getting stiffer after sitting. Seems to be better since we installed new master cylinder lines, but still not as much pressure as there should be. I might just be being picky though, since I've never bled a vehicle with all drum brakes before. It's got a brand new cylinder on it to, new wheel cylinders, hoses, shoes, drums. Still not sure though.

Well, there is ONE Advance Auto store in the whole city that stocks a Pinto master cylinder!  Guess I will start with that.

Yup. That was it. New master cylinder and the braks work better than they have in a long time. Case closed!