Stoping power decreased

Ok, all of a sudden my stoping power has really decreased and I really have to apply pressure to stop the car. When it stops the rotors shudder. I can still drive it, it's not TOO bad but I don't have good stoping power. I have to push the pedal all the way hard to stop the car. I'm sure my rotor's are not warped, but they are old. I checked the brake fluid, that's OK. The brake booster and master cylinder were replaced last year. What could be the problem?