Brakes not very good..... :(

I stopped working on the 71 Satellite long enough to mess with the brakes on the Dart today. When we disconnected the vacuum line from the booster to the intake and blocked it off at the booster. The car will idle fine and I have a good brake pedal. However connecting the line back, when I touch the brakes it will stumble. So I am guessing the booser is the issue.

Anyone know where to find the correct brake boosters for these cars? It is a 71 Dart that has been converted from power drum to power disc brakes off a 73 Dart.