A hydraulic system has a hydraulic actuator for moving a door leaf of a door installation between open and closed positions thereof. A reservoir has a volume for containing a hydraulic fluid. A fluid circuit provides fluid communication between the hydraulic actuator and the reservoir. The system also has a pump for pumping hydraulic fluid from the reservoir into the fluid circuit to operate the actuator. A fluid circuit flushing system of the hydraulic system operates to flush a volume of hydraulic fluid from the fluid circuit back to the reservoir without moving any door leaves of the door installation. The hydraulic system may be part of a mine door installation installed in a mine passageway and having at least one door leaf. The hydraulic actuator is connected to the door leaf for driving movement of the door leaf between its open and closed positions.