W126 ABS pump activation to change fluid

[LandYaghtLover]

I am not even sure if its required. On my Mark VIII, it was since so much fluid remained in the pump.

In short I would like to consider upgrading to DOT5. Even if that is not practical, I would like to make sure all fluid is out when I install new calipers. Since I have two new front units with new soft lines, I am considering picking up rear calipers and just flushing the whole system. Master cylinder is maybe 2 years old.

Thoughts?