hi
I was reading your post and it reminded me of a problem I ran into on a 55 chevy I was doing for a customer...I see now that your problem was the master cyllander
however alot of people forget to preload the booster....meaning when you set up your booster the rod off the pedal should depress the booster 1/8th to 1/4 of an inch with every thing tightened down...this is called preloading and it allows for a firm pedal
maybe that will help you in the future.