Originally Posted by ringoism
2. In the rain, the rear brake shoes get extremely sticky and lock suddenly on light application... and they don't unlock when releasing the pedal, either, or until the bike is rolled backwards a few inches - very dangerous, a good way to get rear-ended out on the road, or to loose control on slippery/loose surfaces. Springs and linkage fine - it seems to be an issue of lining material.
I think your brake shoes need replacement based on the symptoms you described - I've experienced the same problem.
The Impulse brake shoes wear out rather quickly and they are hard to source. The Hero Splendor rear brake shoes are fortunately a direct fit though and cost less than a hundred bucks.