Remember if you are flushing the brake system and are pumping the brake pedal to push the fluid out of the calipers, do not push the pedal any farther than it normally travels. I have seen the "over extention" of the pedal to cause the master cylinder to fail internally, old seals against rough a surface tears the seal.
Flushing the fluid is a good idea and should be done every other year.
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop