[Dukedesmo]

Sounds like the age-old Ducati back brake problem.

On 916 type bikes they need regular bleeding, I suspect it's to do with the master cylinder being bolted to the exhaust mounting bracket onto the engine block boiling the fluid? as the, almost identical setup* on the Monster is fine.

That said even when bled they aren't up to much and I always have crossed fingers when on the MOT brake tester.

* by almost identical, I mean the same master cylinder and caliper, significant differences being master cylinder mounted on footrest (so not heated) and rear disc is slightly bigger, accounting for stronger braking.