Does it do this if the Z axis is stationary with the power cut? or only when moving in feed etc?.
If this is a recent thing, I would still suspect the brake, but if it has always done it, it would suggest that it requires a counter balance, especially if you were to release the brake when the power is off and the head would move down considerably, then it would confirm a C.B. is required.
Most large mills have CB in place.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.