Brake fluid flush, what do you do?

Remember there is a difference between gravitation and siphoning. The former will usually work for most vehicles because the reservoir is the highest point of the hydraulic braking system. Once you open a bleeder, liquid must eventually travel thru the system - assuming no blockages or restrictions. Siphoning requires a 'sealed passageway' (ie: tubing) to allow fluid (ie; gasoline) to travel uphill - - before it moves back down (yes, with some gravity assist) to exit at a point that must be lower than originating supply vessel. In other words, the 'tubing' length from entrance to 'summit' must be shorter than the section from 'summit' to exit - in order for siphoning to take place.