Originally Posted by
cxwrench
Undo the cable at the brake. Pull on the lever while holding the cable in your other hand. This will immediately tell you if it's a cable/housing problem. If it feels smooth and there's no drag check the caliper itself by squeezing the arms together, they should snap back quickly and smoothly. If that's ok it might be a pad wear problem like
cyccommute posted. It's not hard to figure any of this out if you just look at it logically. Break the system down into its separate parts...lever, cable/housing, caliper/pads.
While I agree, I would probably check the pads first. They are the easy low hanging fruit. Cables next, then tear apart brakes. It’s usually the cables that are the problem but it’s always good to start with the easy ones.