Finally got my '85 500SL braking problem solved. Replaced the calipers, pads, drag links, tie rod ends, and master cylinder. Not sure it was any single thing that was causing the problem but, according to my mechanic, the lack of camber in the wheels makes the car super sensitive to nuances in the road and any play/slack that might be induced from one of the above. Also went ahead and replaced the steering box due to the fact it had been adjusted all the way out.
May have done more than what was required but all the parts appeared to be in need of replacement. At any rate, I'm comfortable that I now have a good braking system.