How do you test a brake booster? What are the symptoms of a failing brake booster? I had a leak on the M. Cylinder around 4 years ago. It was bad enough to eat the paint off the booster. I still have brakes but I have a squeel on the rear passenger wheel that wont go away. I already changed calipers and pads. Could this be a symptom?

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles