AFAIK, with MB, you change the pads then the next time you change pads and rotors. At least that is what my shop (Gilly) tells me. I still think your issue is a coincidence issue. Maybe you could take apart your calipers and regrease them and clean anything moving. I always wire wheel the slid pins and put the best grease I can find in them. Also I flush brakes as per MB spec (once every 2 years, IIRC) In fact, all my vehicles always get fresh fluid every year.
I still think it has something to do with the booster. Could you hook a vacuum source to it and see if it gets easier? If so, you might ahve a bad source somewhere.