It can take a long time pushing fluid through a new hose,or even prime the master cylinder if empty, doing that by hand on the old GS meant pumping 3-400 times.
Not too good on master cylinders slamming the piston inside the cylinder when pumping, my method, same as recommended for most cars (and little known...) is to put something on the handgrip to avoid that.(2x4 under the pedal for most cars)
What worked best for me was to rig some suction on the line, rubber hose and large syringue,and with the lever depressed fill up the hose/master cylinder by sucking on it keeping the reservoir full all along.You may have to repeat that a few times til the fluid comes out. Then remove the "rig" and trying not to loose/spill much fluid quickly reattach the hose. Then proceed with regular bleeding.
There is also them vacuum extractors/brake bleeders, I just bought one for similar situations, cheap at Harbor Freight, but haven't tried it yet. With those you can suck right at the bleeder, syringues are too wimpy for that, but note here that Vacuum Bleeding is not recommended by BMW on the later ABS systems, I think yours is OK but check the book first.
Once you have done all that, you may want to attach the lever to the handgrip with a Zapstrap and leave it for a day or so, then inspect for leaks. It took that long for one to show up on my bike after a full line replacement. Much easier and safer dealing with that in the shop than "on the road". That is also the trick to remove all air that could still be trapped inside the master cylinder, they are notorious for that as in.....hard getting all the air out of them. With the lever attached the air bubbles will rise and eventually come out through the reservoir.For that last part the reservoir has to be at the highest possible position, usually achieved by turning the handlebars, but sometimes the handgrip also has to be turned depending at what angle it is installed originally.