Clean8485 is correct about a bad booster giving a harder pedal. That indicates a lack of engine vacuum or internal vacuum leak in the booster and you are fighting the big spring inside the housing as well as the brake system. However on 2 seperate cars I've had a condition where the pedal would slowly drop all the way to the firewall and you would hear a loud WHOOSH sound as it went down. Changing the master or check valve would not cure it, only changing the booster. I can't find any reference to this in any book or manual and it's driven me nuts for years. If anybody knows what causes this I would love to learn.
