Everything here tries to make it look difficult.
To start with, you need a suitable handlebar left hand master cylinder. This is the difficult part. It is likely that a clutch master cylinder will do, but the probability is that the bore will be too great for a brake. Therefore, one will need a clutch master cylinder complete and a matching front brake master cylinder. (Ideally off a crashed identical bike)
The clutch master cylinder will have to be sleeved to take the piston from the brake master cylinder, with 2 o rings on outside to seal it. Having pressed in the sleeve and rebuilt the clutch master cylinder as a brake master cylinder, it is then piped to the existing rear brake master cylinder.
The remote reservoir for this is discarded. A new inlet connection is machined or brazed up to take connection from the new handlebar master cylinder.
The rear brake may now be operated either from the handlebars or by the original rear brake pedal. Either will work it. They are in series.
Myke
Cheers Myke, thats definitely the ideal plan. That part on the previous page, from the Stunt Bike company would eliminate the need to machine or braze a connection into the rear master cylinder as its a bolt on part.
The rear brake lever from the new XADV seems the best solution for a new handlebar master cylinder.