The brake line that runs from the splitter on top of the differential to the left rear wheel is bad. What size is that line and can I purchase a pre-made replacement at an auto-parts store?

 

You'll have to remove the old line and measure its length.. then buy a new straight line, bend it to match, install it and bleed it.

Most likely it's 3/16" line.

I also recommend getting one a couple inches longer, as your bends aren't going to be as tight as the original ones.

Be careful not to kink it, and no you can't purchase pre-bent lines.