Does the pedal travel all the way down? Or is it hard to press the pedal?
-Check your fluid level.
-Check system for air.
-Check for fluid leaks.
-Check your brake pads. Do they squeak excessively, or make a grinding noise? May need pads and rotors.
-Is the brake booster working properly? Is the vacuum line connected?
-May also want to check the Brake master cylinder.
-May also want to inspect calipers.
I'm sorry, but I work at tire kingdom and i cannot tell you how many people neglect their brake systems. Even when you recommend things for their OWN SAFETY. This basically outlines a whole brake systems check, but you might as well do it.
It may just be low fluid and/or pad replacement.