Brake problem 97 316

The local BMW main dealer recently changed the brake fluid on my Gf's car as part of a service, two days later the brake pedal suddenly started dropping
to the floor and felt as though the master cylinder had failed. i.e. if you braked hard the pedal would work normally but if you braked slowly you could push the pedal to the floor. I replaced the master cylinder and pressure bled the system but I still get too much pedal travel if I brake softly. I read that the ABS DSC system needs to be active while bleeding so I went out and activated the abs several times then re-bled the system but to no avail. Any clues as what I can do to get a hard pedal back and what might be the cause of this problem would be gratefully received.
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