Hey Kia,
Yes you can use any rear master from any GT bike or GV bike as it is the same point – but different hose length, so keep in mind!
Shame there is no rebuild kits pre-made to save yours but , it sounds silly, try to replace the brake hose & the washers for banjos , make sure the holes for the banjo bolts are meticulously clean.
When i bleed brakes on client bikes, sometimes they still leak air because the washers need changing (again!) due to micro particles on the surfaces where washers sit on and the bolt needs to be tight (but not too tight!)
// Meditation doesn't mean you have to sit still....