On a non-Mercedes car (a Subaru) I had the same problem. The piston would not budge. I cracked open the bleeder... and the piston could then be moved with finger pressure.

Your mileage may vary...

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1977 Mercedes 240D. 4-speed transmission. Windows, A/C, and sunroof: all manual. Medium Red. 204K miles.
1988 Subaru GL wagon. Dual range 5-speed 4X4. Daily driver w/ twin parts car.
1994 Subaru Loyale wagon. 5-speed 4x4
1966 International 3/4 ton 4X4 truck. Ooh yeah!
1952 Ferguson TO-35 farm tractor.
1984 Snapper rear engine riding mower. Just like Forest Gump.
1988 Nimble 20 sail boat
--Older stuff is built better--