Yep, it's a single circuit. I second 97 SL320, go through the whole system and update old parts. The MC looks like it's well past its prime, just from the outside, I wouldn't question replacing it since it's critical. Brake fluid in the reservoir looks really brown and nasty. Should be change annually or every 2 at most. DOT 4.

Will it solve your problems? Some of them but you may need to send out the booster for a rebuild to get everything right. Also, if your brake fluid is that old, you'll want to flush/replace the clutch fluid.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6