I think it's a general design fault in the way that the rear caliper is mounted in relation to the master cylinder and the run of the hose.
The SS has one really demon sharp rear brake and has never required bleeding, yet it uses the same system components. The difference is that the master cylinder is mounted vertically and not horizontally, and the caliper is mounted above the line of the m/c as it sits above the rear axle and not below.
I know the monsters use this configuration too, and also the 999/749 have gone back to mounting the rear caliper above the rear axle, but retain the horizontal master cylinder - do these suffer the same? If not, I'd be tempted to deduce it's just the location of the caliper.
Mine got MOT'd at the weekend. Not so easy to sneak under the radar now with the new computer methodology.
My major worry was the back brake as it really is useless, but it did stop the rollers eventually with a good stamp and when the numbers were fed in to the computer it said PASS ! ! ! ! Mightily relieved!