Martin- the carbon like stain might actually be seal residue impregnating the
ground surface of the bore due to excessive heat having reacted with the brake
fluid.
I would take a pair of inside micrometers (borrowed from any tool shop) and
determine if the bore is cylindrical (round and straight).....if so, the
stains are only superficial and won't affect brake function. clean them up
good, hone them and rebuild.
If there is a definite ridge or inconsistency in the bore, I would chuck it.
For what it's worth.
Bob Stahlbush
60 TR-3A
66 MGB RD
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