I eventually got a master cylinder but it took two trips. The first one I got was off a 220b, without the power brake servo. It had the wrong number of line connections and a different bore size.

I'm not sure why I grabbed that clock. Maybe I thought it was neat looking and the hands were (and still are) bright and unfaded. I think at one time I wanted to mount it in a nice block of wood as a desk clock.
(I recently saw one that had been converted to a desk clock, with a quartz battery movement, on Ebay for about $150!)

The reason for seeking a used cylinder was that, while my Dad was driving the 220SE, the brake master cylinder broke off of the firewall, causing him to run into a trailer hitch on the back of a Chevy Vega. I also got to straighten Dad's grille and his left front fender. It turns out that, on the original brake master cylinder, one of the mounting ears had been broken for a long time. The broken cylinder had been hanging by just the one remaining ear and nut!

The correct way to refurbish one of these master cylinders, if the bore is badly damaged, is to bore it oversize and install a stainless steel sleeve insert of the correct size. White Post Restorations, near me, does this but I think it runs about $250. I doubt you could find oversize pistons and seals to match the oversize bore and a bigger bore cylinder would requre heavier pedal pressure to stop.

It may still be possible to rebuild your old cylinder. Brake cylinders that have had regular use are less likely to be badly rusted or pitted. (But DO check the mounting ears for cracks!)

Happy Motoring, Mark

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DrDKW