It sounds like a brake master cylinder issue but I am not familiar enough with Mercedes to say that it would be the master cylinder for sure. Try bleeding the brakes following Mercedes bleeding procedures and using approved brake fluid. Are there any signs of dampness at the back of the brake master cylinder/brake booster area. Is the brake fluid clear and fresh looking?
Thanks for the input. I actually have already purchased a new master cylinder( not yet installed), but am doing a little more research as I have never had the brakes come back so solidly after a cylinder failure. I was wondering if there was any possibility that it may have been some type of failure in the ABS control box. Thanks again for your input. Gary s