I have a vacuum brake bleeding set which will draw all of the fluid down through the system until the new appears at the bottle , starting at the furthest from the master cylinder at the back then the opposite one then front NS, lastly the nearest to the master. There is a visual difference as the old stuff is DIRTY looking and has suspended gunge. All you have to do is keep making sure the master cylinder reservoir is topped up as it goes down quite fast. Some ABS systems cause a problem as they have pressure valves which might need overcoming .