The air generally does not travel between front and rear brakes, but as mentioned, it would be a good idea to make sure no air is the master cylinder. You could bleed the master cylinder by pressurizing the brake system, and then one at a time, open each brake line from the master cylinder. Make sure to place a rag under the master cylinder, and wash away any spilled brake fluid after you finish. Mercedes dealerships place importance on having a regular brake fluid flush performed, so if it has been a while or the condition of the brake fluid in the whole system is in doubt, I would suggest also doing a complete brake fluid change.

The best way is to use a brake pressure bleeder tool, that make it very easy for a DIY'er to do this. I use a product called Speedi-Bleed and love it, very fast, the quality is top notch, and decent price for what you get. You can find it at http://www.speedibleed.com