I'm not sure what mechanism actually holds the car when in park. My 124s do not roll more than an inch or so when putting them in park. Maybe I'm a bad driver, but I've probably not used my parking brake more than maybe five times ever.

The only thing that comes to mind is if the shifter bushings are worn or gone, perhaps it's not shifting fully into park. Does the shifter have much fore and aft play?

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2012 E350
2006 Callaway SC560