As you say the fluid just dribbles out of the rear output, so apparently it's not building any pressure there. That will also be the reason you don't get fluid from the rear calipers. On LHD cars the valve block and MC are integrated so I don't know if there are multiple lines from the MC towards the valve block. But in principle it could be that the valve block is causing it, as that will limit brake pressure towards the wheels when the ABS function kicks in.