Did you check to see if the master cylinder piston is coming back all the way so the rear piston chamber is getting fluid. Check to see if you have fluid pressure at the rear port on the master cylinder. If you don't? Loosen the master cylinder as much as you can so it pushes away from the booster. This will let the piston come back all the way so you get fluid to push. If this corrects the problem take the master off and adjust the push rod shorter . I have seen this many times to be the problem
As far getting a master. Check the Cast number. Rebuild yours or at least keep it if its original, If you don't use it I am sure someone on this site will grab it to get an original for their car. The original mopar master cylinders are getting hard to find.
http://www.moparts.org/Tech/MoparAction/Master_Cylinder.pdf