I took apart the booster in my 65 C-body. It had hard, torn rubber in the diaphragm right where the vaccum port sits, otherwise the rubber was soft. I had installed an in-line charcoal filter on the vac hose to stop gas fumes. I searched widely and didn't find anywhere to buy internal parts. I'll try searching for the place in post #3. I ended up buying a roll of neoprene rubber sheet on ebay and replaced the diaphragm rubber myself, using flowable silicone sealant. It didn't have the folds of the original rubber, but worked when tested. However, I never ran it on the car.
I later got a correct booster/MC for only $70 on rockauto clearance (great guys). In the interim, I installed a used 74 Valiant booster by drilling new mount holes in the firewall (other than stud locations looked identical, same Midland-Ross type). After swapping, I sold that one to guy here. On my 65 Dart, I installed a booster from a 99 Breeze on the 74 stand-off brackets/lever.