I was driving and the brake pedal suddenly became VERY stiff. Brakes still work ok, but they require a ton of effort applied to the brake to stop the car. I kept driving and noticed that while on the highway the brakes feel normal...then return to being super stiff at low engine speeds. Engine also will no longer shut off with the key, I have to use the stop lever.

I removed the check valve, and apparently there is supposed to be something inside this thing? Mine is empty and is just a hole. Does this look like a failed check valve?

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles