No Power at Rear Brake
Change the pads. First thoroughly clean the rotor with alcohol, brake clean, etc, but definately change the pads. If the lever action and power at the lever feels good, and there is just no power to the brake, it is "definately" the pads. We come across this problem very regularly at the shop and 90% of the time this is the problem. One more thing to check is that the mounting bosses for the brake are perfectly flat. If the caliper is running cocked it will drastically affect power to the caliper. Take it to your shop and have them check and if necessary machine the mounting tabs to fix this problem.