I've flushed ABS brakes in all sorts of cars, MB's included with 15PSI. Where you can sometimes get into trouble is BLEEDING brakes. Occasionally after major hydraulic work where the system is filled with air in one end or the other, it gets stubborn and takes some pressure bleeding before or after using the two man method.

Flushing with 15PSI will be no problem as long as you don't let the fluid level get too low.