I bought a used bike a few months ago, and I've noticed a drastic reduction in brake power.
At first, I was easily able to lock-up the rear wheel while squeezing the brake lever only half-way to the handle bar.
Now, I can squeeze the brake lever all the way to the handle bar and I'm experiencing negligible stopping power. I've shortened the brake cable at the caliper several times, and it's now as tight as I can make it without causing it to close slightly and rub the rim. Squeezing the lever slightly causes the brake to close and contact the rim, but I can continue squeezing all the way to the handle bar and still not apply any real stopping power. It feels as though the cable has become more elastic with time, like the whole rear brake system is softer to the point of being nearly useless.
Is this "normal"? Is it an indication that it's time to replace the cable? Is replacing the cable likely to make my brakes more responsive?
Upon closer inspection, my brake pads are worn down to plastic-on-metal; there is no rubber left at all. This is very likely the cause for my perceived soft braking, and definitely the cause of the lack of brake power.