[twindman]

I had one of those cheap black plastic devices under the sink. Can't remember but something went wrong. I just disconnected it and took it apart - easy job back then. It is nothing but a tube with a marble in it. When water flows to the tank, the marble hits something that stops it from plugging the outlet. When reversed there is a perfect fit for the marble to stop the backflow. I just took the marble out and put it back together so I didn't have to do any refitting with the pipes.