Brake lines on the older Expys tend to rust. I had a similar leak and ended up replacing the line from the box to both rear tires. Lines were pretty rusted and I am lucky they did not collapse while on the highway. My front lines are not nearly as bad but I will replace them in the near future.
Luckily, there was a DIY shop not to far away where you could rent a bay with a lift by the hour and the rental included access to tools a mechanic and on line repair guides. Not too bad of a job. You can buy the lines pre-bent and you just attach them together (expensive) or you can build your own using brake tubing you can get at any add auto parts store; they are pretty easy to bend or you can get the bending tool for a few dollars. Be sure you have all the components, couplers and adapters you will on hand so you don't have to run to the parts store; NAPA seemed to have the he best variety. Having a lift is not a must but it sure makes the job easier.
