40 miles isnt breaking in anything. Discs need at least 200 miles of strong riding in order to be somewhat broken in. I can lock up rear on my road bike with MX4‘s, so if BB7 wont do it, then it is not set up properly or not broken in.
Disc brakes have lower initial bite then v‘s, but braking is more consistent up to the limit when the tire starts to loose traction. V‘s tend to grab quickly and proceed even quicker to lock-up, they are mostly an on/off brake. Discs on the other hand dont have such an initial bite, but they scrub speed down much more quickly then v‘s, because you can balance the braking with them right up to the point where tire almost starts to skid, but not yet - and that is the point of max deceleration, efficient braking that scrubs the most speed in the least time.