Emptying the old brake fluid from the system.

The car has double master cylinder system and its is diagonal brake system. Front right+rear left, front left+rear right. On the front -disk brakes and on the rear-drum brakes.
i have question that related to the emptying of the brake fluid before adding new fluid.
1)i begin(beginning with the front wheel as the manual said) to empty the old brake fluid with an assistant that push and release the brake pedal while I open and close the bleed nut until there is a moment, that the fluid stop to get out(in the resevoir, you can see, that one part of it is empty). After that I do the same to the other front wheel, till the fluid stop get out and the resevoir got empty.(As I said, it is diagonal system)at this moment, is it mean that all the old brake fluid that was in the system included all the lines to the all wheels from the master cylinder gotout completely?Is it mean that emptying the fluid at one front wheel, also take out the fluid from the diagonal rear wheel line?
(because when I continue to the rear wheels, it seems that no more fluid got out.)
or the lines, after the master cylinder, are still have brake fluid, only that the master cylinder can't push the fluid out(maybe air that been compressed?) Because the resevoir is empty?And is it only make me think that all the fluid got out?
Please, I do not look for advise how to do the job.I would like, only, to get answer to those specific questions and this situation.
thank in advance