Don’t get me wrong, i prefer to do work myself if possible...esp brake bleeding, where there can be a big difference in pedal feel between “good enough, and besides we’re on flat rate and make more profit if we get the job done in half the time”, and “right” or “as close to perfect as possible”....i tend to like latter. Not impugning pro mechanics, they just are working under different constraints than you are. And since brakes will work fine at “good enough” and 95% of driving public will never notice (heck, with most Americans’ braking habits, the mush functions as a valuable anti-lock mechanism...), it’s not financially feasible for the pro to get air out past this point. On a sportbike where you only have 1 master, 2 calipers, and 18" of hose between em, level feel improvement is super noticeable as you do the last bit of air. Even l;eaving em overnight once you finish and coming back to bleed out all the pinhole-size bubbles the next day produces noticeable improvement. How many pros do you see bothering with that step, or owners who would be willing to pay for it?
the gravity plan is a good one for simple systems where everything flows basically downhill, but ONLY to get initial large volumes of air out after major component removal/rebuild. Get the calipers on, then leave it overnight with top on master threaded loosely on. The next day, if you’re lucky, you’ll have enough pedal that you can commence bleeding for real. You don’t want to try to drive it this way, tho...very dangerous. It just helps get you past the frustrating period at beginning of job where you are effectively trying to force fluid through system but are just compressing the air you’re trying to get out when you depress pedal. You can avoid most of this if you be sure components are full of fluid and bled on the workbench before installing.
However, with ABS system, you have LOTS of additional lines, up and downhill pipe runs and other such which makes the gravity plan not feasible. Look under your hood at the ABS assembly (usu. on pass side, just aft of and below headlight, lots of hydraulic lines running to it) and you’ll see what i mean. Due to same factor, I’ve read (repeatedly and from reliable sources) that you can’t adequately bleed ABS system via old “long suffering spouse pumping the pedal while you lie on floor and absorb vast quantities of brake fluid through skin” plan, and i believe them from looking at system. I don’t think the vacuum bleeders like a n ezi-Bleed or Mity-Vac will work for this either....what you need is a system that injects fluid into the system from above via pressure. Special cap that replaces master cap (or you can make your own from a spare cap) and fluid is forced in, usu via air pressure acting on a large reservoir of fluid. Pro versions of this rig can run hundreds or worse, but there are starting to be some consumer-grade versions since so many cars have some sort of ABS. One uses air pressure from your spare. No experience with those but I’m probably eventually gonna get one.
Finally, the old fluid is part of the problem, so, yes, you ideally want to get it all out of the system front and rear, rather than just draining front hoses. Often easier to start adding new fluid b4 all the old is gone so you’re using fluid to push fluid...there will be some mixing but you just keep it up til what’s coming out of bleeder nipples looks like what you’re pouring in.
Hope this helps some. Sorry for being so wordy, just don’t want anyone to get killed....
bw