You might very well have a flaking brake hose. This will happen sometimes where a flake of rubber acts as a one way brake valve. The hoses are cheap and it would be a good idea to replace them every 15 years or so. Do it at a time when you're opening up the hydraulics for some other reason.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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