The rolling when putting the car in park is normal. Mercedes have pretty thick parking pawls and as a result have a larger window for the pawl to engage in and more travel before the pawl clicks into the window.

The parking brake not working is not normal. The two functions are not related. Park on the tranny functions mechanically inside the transmission, the parking brake functions on a set of drums inside the rear disks.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record