For brake flush I built a power bleeder out of an old lawn & garden sprayer. Pump it up to 15 psi, turn the valve on, and go with it. This makes bleeding and flushing a one person operation.
As far as doing the old fashioned way using gravity, I have two cars that I've recently done brake work on, and bled them with gravity. Trust me when I say, it doesn't always work. One in particular I couldn't get the brakes to work properly no matter what I did. Built the power bleeder, cracked each bleeder loose and the LF and RR both had a couple bubbles come out.
Also worth mentioning....the bleeders themselves. Most calipers the bleeder is screwed in, and there is a possibility for air to leak AROUND the threads. When bleeding or even flushing, remove the bleeders and put some grease or better yet teflon tape on the threads but ONLY on the threads, not on anything else. Some calipers, the bleeder is near the top but not AT the top and on those sometimes with the threads leaking air around them, air can get IN through the threads, creating a bubble or two at the top of the caliper. Some of those you have to partially remove the caliper and "make" the bleeder go to the top by twisting or repositioning the caliper. I know one particular Ford that is like this, and at least one GM car.