You are not alone. Did you do a search, there are many posts on it? Here is a recent:
Some causes:
Not enough brake pedal free play.
Swollen rubber hoses
Sticking brake caliper pistons
Small piston in the brake master cylinder sticking- seems to be one of the more popular causes
Servo/booster, piston sticking
The threads should provide several troubleshooting techniques to narrow it down. Or, if you system has not been rebuilt is sometime, rebuild it all and get it behind you.
Tom