Thanks for everyone's input
As it turns out, i installed the rear calipers on upside down and backwards. As an added bonus, I kinked the rear brake lines, and killed the master cylinder with all of that pumping. And as a cherry on top, the left rear caliper was the wrong type (close enough it "looked right" on first inspection, but wrong enough to bung it all up).

DOH!
I'm going to create a
video post on my 300SDL blog detailing the correct procedures and pitfalls, so other noobs like me out there can learn from this debacle.
Never thought this would apply...
"The fault dear Brutus lies not in the stars but in ourselves"
Wanted this to work out, but I feel like such a yutz.