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Hello, my name is ***** ***** I am a professional here at Just Answer. I have noticed that your question was not getting a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.The problem you are describing sound like you may have a failing master cylinder, all pressure to the system comes from it. I would start with having them recheck it and the pressure coming from it to the breaks.
Yes I would do both, the master should produce pressure as soon as it pushed no matter how fast that is done.
So are you still getting the delay? Even at 400 PSI the brakes should grab.
Something dose not make any sense here. Im not sure what is could be if the master is building good pressure beside the portioning valve hanging .
If the pedal is hard as a rock its is a possibility. In most cases if the booster has good vacuum and its not leaking it will do its job, Are you sure there nothing binding?
The vacuum supply line dose need to be vacuum line but I have never seen one suck shut. Nothing to loose by at least replacing it and see.