OldRover

Working on a 1988 560sl. The problem: currently have no rear brakes (when on a lift and brakes applied, the rears wheels turn). Quick background. Vehicle was purchased almost two years ago. Prior owner had not positioned rear pad keepers properly resulting in squeak. I had replaced the pads and rotors at that time. All functioned perfectly. Recently I decided to do a fluid replacement. During the bleed I did notice the left rear had minimal flow (it took much longer to get the new fluid than opposite wheel and amount of fluid moved was less than half on each pedal depression). At no time did reservoir ever go empty (never below half). After all wheels bled the pedal travel was far to much and soft. I went back to confirm calipers where free. Bled a second time and encountered air. Not sure how since system was never opened and reservoir always had fluid. Currently acting like a vehicle with a bad proportioning valve. I have not worked on these cars since they were new (back in the nineties). As I remember the bleeding process is straight forward.
So question one. What is the proper bleeding procedure? I know that the Land Rovers I usually deal with have a rather long and complex process. If not followed correctly a spongy pedal results. Also, I have seen (and repaired) a number of Rovers that were power bled resulting in fracturing the internal disc valves of the modulator as well as seals damaged. Is there any similar damage that can occur to the MB ABS system or master cylinder if bled improperly? Lastly, is there a diagram of the hydraulic circuit? As I recall the two lines of the master cylinder both go to the ABS unit were each is applied (half) to both fronts and one of the rears. There is no external proportioning valve. Correct?
Thanks in advance.