How do the flexible hoses look? Old ones can cause a bunch of interesting problems. I once let my 210 sit for ~3 weeks while traveling. Upon return the brakes were all mushy. Bleeding it addressed the problem, never to return. Another owner had a similar problem every spring after storing the car for the winter. It was due to very fine cracks in the flexible hoses. Replacing them fixed the problem for good.

This car is at the age where I just go around and replace all four flexible hoses.