Where is this rear resrvoir?
Im having this same exact problem on my 300CD accept that theres play to an extent that does nothing. (i can press the brake and nothing happens at all) but once i get to that point the brake catches pretty strong.
Think it might be something with the brake-booster? i tightend a loose conection today and it seemed to help a little bit, but didnt have time to check it out on the road.

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'83 300 CDT
'82 300 CDT- flooded and locked up, looking for new engine
'81 380 SL - Sold
'86 300 E - small probs.. 60k miles.. great condition overall