I'm very familiar, having changed many sets over the years. If there's no air in the system, pulling fluid away from a hydraulic piston must retract it. It can'

I'm very familiar, having changed many sets over the years. If there's no air in the system, pulling fluid away from a hydraulic piston must retract it. It can't just sit there unless there's an air bubble to expand or so much negative pressure that it forms a vacuum somewhere. Even then, pressure on the atmosphere-facing side would push the piston back toward wherever the vacuum or air bubble is.