73 duster brake question

As mentioned above. But I think I would start with checking the available vacuum you have at the engine. I have power disc brakes on my 69 Dart and thou it doesn't have a real big cam it is noticeable. It has a Hughes Engine HEH2328AL 223/228 @.050 and .534/.555 . It's always had enough vacuum to operate the booster and the car stops really well. I have the stock BBP disc brakes out front and 10 2 1/2" drum in the rear.