Almost certain you have air in the lines. Rebleed the system.

Your mechanic is a little off...or at least for these MB brake systems. New discs and pads will grip fine. It won't be GREAT for a good 50-100mi, but it will still stop hard enough to kick on the ABS. Pedal travel shouldn't be that long

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k