Power brake conversion issue

I had a Toyota truck do the same thing. I changed the m/c but that didn't help I sold the truck because it had over 300 thousand miles but often wondered about that problem. The only thing I could ever figure out is maybe where the vacume hose was attached the supposed to be vacume was positive pressure and it would pressurize the brake system. My pedal was rock hard. I might be way off base but it is all I could think about.