Hi Duke,

The reason I asked is that I was checking my pads and pushed back the piston with a C-Clamp. Shortly thereafter I realized the reservoir cap was still on, so I removed it. I replaced the cap when done, and pumped the brakes.

When the car engine is off, the pedal is rock solid. When the engine is running, the pedal is spongy. I assume air is the culprit?

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black