W211 with SBC
SBC Brake Fluid Flush - Mercedes-Benz Forum
Anytime you can mash the pedal to the floor it points to a fluid issue. I would rebleed thoroughly, and if no results probably drop off at the dealer for the ABS/SBC purge.
Note that if you always (hot or cold) have a firm pedal with the engine off, the problem could lie elsewhere. Booster vacuum leaks often give rough running and codes as well. Also, some boosters have an oring/gasket at the master cylinder junction that gets overlooked when replacing the mc.