Today I took off the left rear brake drum in order to dismount the faulty wheel brake cylinder. I decided to disassemble it to look for the cause of the problematic bleeding screw. The reason proved to be quite simple, the whole of the bleeding screw was simply clogged up.
As I did not know how big the whole was, I first took a small nail and pushed it through the whole. Then I opened up the whole completely with a small drill. After cleaning everything with brake fluid, I assembled and mounted the wheel brake cylinder and brake drum again. After that, we bleeded the brakes succesfully. Since I always use a block of wood behind the brake pedal to avoid pushing it to far in order to prevent damage to the main brake cylinder, I did not feel any pedal resistance at first. Damn, I thought, the bleeding still did not work. In fact, I simply couldn’t push the pedal deep enough because of the block of wood!