It's very common for fuel lines to leak after you replace the brake line. There are 2 fuel lines and one brake line running together along the left side of the car. They are all held by the same clamps. The lines rust under the clamps and when you take the clamp off to change one line, the rust falls off and the other lines leak.

My 85 300SD just went through this. By the way, it takes precisely 15 feet of 5/16 tubing to replace each fuel line on a 300SD.

Here is a thread on replacing your own fuel lines. You might as well replace all 3 lines or in your case both fuel lines as the brake line is new.

I found it easiest on my car, to do the lines in 2 sections with a connection in the middle. When they build the car they obviously put the lines on the body before they put the body on the subframes.

Here is the link to the fuel line thread, there is some useful advice here.

http://www.peachparts.com/shopforum/showthread.php?t=120795