a little bit of vibration can be more effective than static brute force.

what's the state of the lockring around it? or can you not even see it since the piston won't move?

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1982 300TD 210K miles ("The Replacement" aka "The Anvil") - SOLD
1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
1995 Ducati 916 (SOLD, sniff)
1999 Ducati 900SSie (SOLD)