I just had a sticky left caliper on my 1987 300D. When I got it apart to inspect, the piston had rust blisters. And they were small too - it doesn't take much to lock the piston.
So, I found that the prices here on FastLane for new Lucas calipers were reasonable, so I got a pair.
I did both because I decided they had both endured the same maintenance, and had been exposed to the same old brake fluid. Now I have a nice new pair of front calipers, new premium brake fluid, and no worries.