Brakes are not stopping correctly

[Ralph50]

I have repleaced the master cyl 3 times and now finally have a good firm brake pedal. Everything seems ok except the brakes dont seem strong enough to stop the car. A brake torq just pulls the car forward. I think maybe the vac power assist is not working, I did check the hose to the vacume assist and it did have vacuum but I dont know if the check valve is working.
Question, is it important that the master cyl is sealed to the face of the vac power assist, there is a lot of rust on it.

R