Too late. I splurged for an all new replacement Clutch Master Cylinder and installed. Out of curiosity the original unit was disassembled for inspection. Its machined cylinder bore ( 5/8″ ) had no corrosion and was therefore in fair condition. The residual fluid however was filthy black with contamination. The annular lip seals on the piston plunger were likely perished and my thinking is that I could have replaced these and saved the core unit. It may present a small challenge to find these small tapered sealing lip rubber bits. The pictured piston assembly resembles (copies?) that of a type of piston and valve frequently seen for early British Cars made by Dunlop and Girling. One closer look of this piston notes some scuffing. A proper rebuild would require honing the bore. Previously skeptical of dealer recommendations for annual brake fluid changes, cooling system flushes and similar ilk; I now have first hand evidence for the justification. Brake fluid is hygroscopic and thus the degradation. Why not silicon based DOT 5 instead, which is hydrophobic? That is another debate. Perhaps the *new* complete assembly was the path of least resistance after all.