Yeah, this was my first time bleeding brakes, and I just learned the block trick. I had read about the problems with pressing too far, but I forgot to warn my partner until a little ways in to the first brake. He's not the most coordinated individual in the world though, and I'm not sure what exactly he did when he pressed the pedal at any point before or after my advice. Hopefully, just one failed attempt at bleeding 1 wheel before quitting to go buy a mityvac didn't damage the MC. I suspect with that much missing fluid...it was a LOT...that reusing the seals was a really bad idea. They looked okay, but they very well may not have seated properly or could have been damaged without me noticing it. I haven't checked for signs of a leak yet...still recuperating to be honest from all the work I did on the car..., but I will check later today or tomorrow morning. I went ahead and ordered the rebuild kits along with a new bracket for my sway bar that had cracked on general principle after reading threads, and if that doesn't fix the problem I will look into a master cylinder.

My one fear is that it has something to do with the ABS or SRS systems. Does anyone know if you can actually bench bleed the master cylinder for this vehicle without the ridiculously expensive (several thousand dollars) Mercedes special tool system? Also, I read somewhere that when working with a vehicle with ABS that you should depress the pedal 25-30 times before bleeding in order to...I want to say pressurize or depressurize the ABS system. Anyone know if that is correct?

For those who are curious, apparently when the bleeder is open...or hose detached for that matter...there is more travel in the pistons inside the master cylinder. The pistons can go further than before, and they might come across some nasty bits of corrosion or even just gunk that have formed over time in that section of the housing due to lack of use. When the seals come into contact with this section they tend to tear which causes your master cylinder to fail...thus causing increased pedal travel & spongy feeling.

I looked online & discovered two things about master cylinders. The first is that, due to liability issues, very few places sell a rebuild kit anymore. The second is that the one rebuild kit I could find was actually more than a lot of the new master cylinders I found online. Granted these were some of the cheaper brands, but if you repair the seals you might still have a badly corroded housing. Same with buying rebuilt MC's. I may be wrong, but I think I'd rather have the cheap new MC over a rebuilt one. A new good MC was not that much more than the rebuild kit either.