How do you disconnect left rear rubber brake.

You'll need two wrenches, one to hold the metal line steady and the other to unscrew the rubber line. Once they are disconnected there is a clip that holds the rubber and metal lines stationary. Use a flat screwdriver and/or pliers to slide this clip from its place (it may be kinda stuck due to rust, dirt, etc). Be very careful not to tist the metal line while disconnecting it from the rubber line.