Originally Posted by JARblue

This is wrong. The fluid does not mix in the lines except for very small amounts, which is bled out. With a power bleeder, the new fluid is pushed through the system forcing the old stuff out. With a gravity bleed, the new fluid is also forcing out the old stuff, but it is working solely by gravity and takes a lot more time. Either way, there is no need to run any more brake fluid through that line once the new stuff starts bleeding out. You're just wasting good brake fluid.

I've never needed more than 2x 500mL bottles of Motul RBF600 for a full brake flush, but I always buy three and use half one to flush the clutch system