The 1983 R80ST has a single front disk brake. The brake disk is mounted on the right side of the front wheel. This bike did not have the front disk, so I got a new one. I rebuilt the front brake caliper and you can see how I do that here.
Here is the description of how install the front brake and a summary YouTube video of how I do this work.
VIDEO: 1983 BMW R80ST Install Front Brake