I use to work for a Hyosung dealership and the rear brake master cylinder is a problem. They will work for years happily on there normal stroke range but push them further that they normally move and they rip the seals to pieces. Use to see them all the time when a new tyre was put on when when the tyre fitters have pushed the pads back then tried to pump the brakes back up as the plunger moves further than it normally does. try bleeding the brakes with your fingers over the end of the brake line first just in case air has air locked the master cylinder as having your fingers over the end of the brake line gives the master cylinder something to work against but the fluid will force it's way past your fingers when it gets there. if you can't get any pressure it's time for a new master cylinder as Hyosung didn't sell a seal kit when I was working for them, might have changed now hopefully.

hope this helps