As Chris notes, forcing old fluid up into the ABS unit is not good because it carries crud with it. Crud settles into the calipers because they are the low point in the system. Forcing it back into the ABS is the same as adding dirt to the expensive ABS.

But I doubt you did harm: the bleed screw relieved most of the pressure and allowed fluid to depart. Next time, wrap some vice grips in tape and squeeze the line enough to just clamp it shut. And flush your fluid every two years.

To bleed or flush, get a pressure bleeder from IPD. If you use the pedal technique, make sure you do not press more than two inches down (less than full pedal travel) so the m/c seal does not travel into potential worn or corroded territory inside the m/c. Mark where the pedal goes at full travel when the system is working and then don't go beyond that when bleeding or flushing.