when you flush a system all you do is suck all the old fluid out of the resevoir with a suringe of sume type and top it up with new fluid, then you go to the furtherist bleeder and open the nipple and pump the peedle till the new fluid comes out then go to the next furtherist line and carry on till new fluid is coming out the closest line. BUT ALWAYS KEEP THE RESEVOIR TOPPED UP!! you dont want to get air in the system.
thats all you do to flush a brake system if you got ABS then you gotta do the ABS system sepreate.
good luck its easy to do aswell basically just a full bleed of the system.
and all old fluid will be gone.
my 2 cents worth
oh and i am a mechanic and thats what we do at work