This has been a frequent and frustrating issue for me. I sweat a lot and much of that sweat seems to drip down into the entrance of the housing for the rear brake cable under the top tube just in front of the seat tube. I have to watch that the cable and housing don't get corroded. An occasional squirt of WD-40 into the housing there helps, but if it gets too bad, you'll just have to replace the cable and housing.