Like what Torque said, it's probably your brake pads wearing out or the wear sensors themselves are shot as well.
I also highly recommend replacing your brake lines with a braided stainless steel line as well. They do not flex as much internally under braking pressure and also last a lot longer than the OEM rubber lines. They also cost about the same as well which makes it a cost effective upgrade.
You will also notice your brake pedal feel is much firmer