About two years ago, I adapted a brake fluid specific Magura clutch master cylinder for left handed rear braking (LHRB) by connecting the new master cylinder directly to the rear brake caliper on my RevLoc equipped '04 KTM 525 EXC. This setup has performed superbly since then and provides excellent progressive two finger rear braking. The reason that I used the special brake fluid compatible master cylinder is that Magura claimed that the OEM mineral oil clutch master cylinder was not compatible with brake fluid and that the rubber seals, etc will deteriorate.
In the interim, I have run across at least three long-term cases where the OEM mineral oil master cylinder has been successfully incorporated into the LHRB system without any problems whatsoever with the seals. Therefore I decided to give the OEM mineral oil master cylinder a test in my LHRB setup just for kicks.
After thoroughly cleaning and flushing out the mineral oil from the OEM Magura master cylinder, the whole project took less than 30 minutes to complete including reverse bleeding with 5.1 Motul brake fluid using a syringe. The easiest and least expensive way to connect the stock master cylinder to the rear brake caliper is to connect the stock (flushed and cleaned) clutch line to the stock rear brake line via a "double banjo" fitting. These "double banjo" fittings are used on most street bikes to split the single front brake line to the dual front brake calipers and are available cheap from any motorcycle salvage yard.
Just run the original clutch line in roughly the stock location along the left side of the engine. The connection to the rear brake line will be near the oil filters (in KTM's). Note that the double banjo fitting is not a single hollow bolt joining the two banjos piggyback, but rather it is a unit about 1 1/2 inches long to which the two banjos are independently attached. Also, the original clutch slave must be blocked off. It isn't too difficult to make your own block off plate, however RevLoc and I believe also Rekluse sell these block off plates.
Results: The same superb braking.....perhaps even a little better since the project involved replacing a 10.5mm master cylinder with a 9.0mm unit, which provides increased "leverage" making two fingered braking now even easier. IMO, the LHRB is an essential complement to the auto clutch especially so for technical trail riding, steep downhills and tight switchbacks.
Note that this procedure is suitable for all auto clutch equipped bikes that come with a stock hydraulic clutch master cylinder.