On certain vehicles, the surface finishing of the piston in the rear brake is improper. If left unfixed, the cup on the piston can wear causing brake fluid to leak, and in the worst case, the brake stops working.
The two pistons and two cups for each rear wheel should be replaced with the factory approved replacement units. In addition, if the brake shoe is found to have brake fluid on it, it should be replaced with a new one.
yellow paint on the back plate