NONE of the posts I've read re: 500e500's brakes issues have indicated (w/in regular use) brake failure of this sort...
In either case, I'll start a generic list of possible causes of brake issues:
>low brake fluid
>leaking lines/ calipers
>not kosher= resurfaced rotors vs. new units
>non OEM parts
>crimped line/lines/ non oem parts
>brake booster failure
>brake master cylinder failure
I'd suggest taking it to mech. pronto!