A couple of weeks ago our Granddaughter was driving a 2013 C300 MB on the freeway. When she attempted to slow down the brake pedal went to the floor. A short time later the brakes returned to normal operation. As of today 9/04/2022 a repair facility could not find an issue with the brakes or any indication there was an problem Now there is a useless vehicle in the driveway.
You don’t say what kind of repair facility this was . Either use a dedicated brake shop or the Mercedes Dealer and expect to pay for the diagnostic .
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A brake master cylinder that’s leaking internally will cause the brake pedal to sink to the floor, and then start working again.
Tester
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OP said pedal went to floor. I hate it when that happens.
See above. Sometimes you can pump them up to work enough to get home.
I’m presuming no dashboard brake or ABS related warning light.
Another possibility, brake fluid overheated. If it gets so hot it boils, symptom is brake pedal goes to floor without stopping vehicle. Was granddaughter going downhill and/or using brakes aggressively before this occurred?
Whenever I’ve had brake master cylinder problems, symptom is brake pedal slowly goes to floor when holding it, usually noticed like at a stoplight raather than when driving. When driving typical brake application is too brief to notice brake MC internal leak problem.
No harm done to ask shop to check for pending diagnostic codes in the car’s ABS system.