'70 Challenger Disc Brakes

[RabidScott]

  The brakes are shot on my '70 Challenger.  When I try to depress the pedal it goes to the floor and does almost nothing!  They're manual disc brakes.
  There was a spring pulling the pedal off the brake light switch, and the the was an extra brake pad backing-plate (the things brake pads are bonded/riveted to) behind the brake pads like a spacer.
  Also, I tried to replace the studs on my '70 Challenger, and found neither what the local parts house offered did not work.  Also, are the OEM brakes pin type?
  This whole thing is driving me crazy!!  I won't drive my Challenger until the brakes work correctly!!!  What the hell is going on?  What would you replace first?  The master cylinder?  Could the proportioning valve be a suspect?

[BIG MIKE]

do you have fluid int the master cyl.? low pedal is most commonly caused by

air in the lines,
bad master cyl.
 line broke,
rear brakes out of adjustment
wheel bearing very bad ( the wheel would be leaning)
caliper suck to pens but the piston is free

[RabidScott]

JUST AN UPDATE!!

WHOA DAMN!!!  I think I'm lucky to have made it home!!  The entire rear master cylinder circuit was bone dry!  The master cylinder held liquid in the reservoir, but it wasn't in the disc brake lines!  

The brakes have been upgraded to 11.75 rotors ('79 Cordoba pin-type) and '73 A-body spindles.  A new master cylinder wraps things up.  Tonite I finish bleeding the rears, and we'll see how she brakes!  Lets hope a tree (or new Lexus!   ) doesn't stop me!