The rear brake on the multi is notoriously shitty.
Part of the reason is the reservoir and it's rubber hose to the master are sitting right above the hot exhaust.
The ABS pump is the highest part of the system, and from what I've read, even with bleeding it thoroughly, air bubbles can get trapped up there. The in and out lines are at the very top of the ABS pump, and I read at putting a banjo bolt with a bleeder at the top of the ABS pump can help get those last little bit of air bubbles out.
Of course this could just be a total waste of time and effort.
I am trying everything I can to make the OEM brakes as good as possible without spending $$$$ on new calipers / Master / rotors.... Motul 600 fluid, EBC double h pads all the way around, resurfacing the discs, bleeders on the ABS pump and bleeding the crap out of it, and I got a special high temp hose and insulated jacket for the rear reservoir to master hose.