Okay, this problem developed around 2 months ago. When I'm stopped at a light I will have to brake pedal pushed down until firm. Seconds later my car will start to slowly creep up and I have to push the pedal down even more until it gets firm again, in which the pedal is basically towards the floor. After that happens if I immediately release the pedal then pump again it gets firm with little pedal travel at all and doesn't lose pressure. My fluid level and pads are fine so what could the problem be?

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500