Purging air from a hydraulic system is necessary when __________.

Correct Answer: Option D — the system has been drained and then filled with new oil

Air trapped in hydraulic systems causes spongy operation, erratic actuator movement, and cavitation in pumps. Purging is mandatory after draining and refilling the system to ensure all air is displaced by fluid, restoring precise control and preventing damage to internal components from cavitation or aeration.