Aside from proper bleeding, be sure that is your problem. Once you have no air coming with the fluid, a proper break in of the pads might be necessary. MBs are fond of having the brakes feel spongy after pad replacement especially if the old discs are reused.
Until the pads are properly worn to accomodate the disc irregularities the pedal pressure is higher than normal (for a similar stop) causing a spongy feel
Bosch Master, ASE Master, L1
33 years MB technician