The fourth, green, line goes to your climate control. For new parts try the buy parts link or call. You probably want the X-connector (it's not a cactus). Possible p/n 1170780045

If you want to troubleshoot the brake vacuum completely, you could block off this line and test your brakes. You'd lose the functions on the line: shutdown, central locking, climate control. But unless there's been some significant mods, it shouldn't affect brakes.

The lotus forum has an excellent cutaway of a brake MC and description of the "falling pedal" that could be caused by small internal leaks in the MC. I don't know if the MB MC is the same or similar enough, but the engineering seems plausible.

http://www.lotustalk.com/forums/f101/brake-pedal-sinks-after-while-21866/

Check the grounds for corrosion for your "quite" horn. There are also carbon brushes in the steering column stalk switch, or the contacts themselves behind the horn pad. Or a bad horn...