Stiff pedal when applying brake

It has been a while since I have this hard pedal brake problem but I just let it stay as it is. Maybe I used to it already. I know it is dangerous but I know how to handle it already. Went to check with the mechanic about this and it was diagnosed to be a broken brake master. So we changed the it but the problem still exist. Unsatisfied, went to another car shop and they changed the hydro-vac. Still, nothing happened. As far as I could remember, I changed the brake master and hydro-vac twice but the problem remains. I have heard from my friend that the vacuum/air from the alternator is not working properly? I checked it myself, and found out that there's a small hose from the back of the alternator connected to the hydro-vac. This must be the thing that my friend was referring to. Does it contribute to the hardening of pedal break if the vacuum from that hose is weak? I planned to have the whole braking system checked first before replacing another. By the way I am only using a Japan made replacement. What is really the cause for this?