Mark,
Most riders do not know how to use a rear brake on a motorcycle (me included). On the street, it is difficult to go fast enough to use the rear brake through long corners to stabilize the bike. The trick is to adjust the brake to never lock up during the application of maximum force. This should give you a decent stop on the street when using all the brakes. Using the rear brake with the front at all times is not a bad way to go. This technique works very well with Girdraulics. It will prevent the same problems that the new steering stems prevent. It is best to have a new steering stem and not rely on the rear brake because you may not be able to deploy the rear brake for a number of reasons (panic stops, the rear brake adjustment went away, the shoe broke, etc.)
An 8" brake would work fine on the rear and it provides much better braking. Pat is putting one 8" on the rear of his bike for better braking.
The Vincent rear brake is already detuned by design and it will not work as well as the front. The brake plate and shoes are pretty poor and the linings are low friction. It may be that you can tune the rear brake up to do what you want, but it is not clear to me what you want.
Bill Lomas was the first GP rider that I know of who perfected the rear braking technique of maximum rear braking with no lock-up. He kept it a secret. When Bill and John Surtees were at a GP event, John complained to Bill that his Flash had a wobble at a certain part of the track. Bill took out the Flash and found nothing unusual. Of course, Bill was probably using the rear brake out of habit, which would have gone a long way to preventing any wobbles. Surtees might have been very close to finding the answer he was looking for.
David
) you would want a more efficient rear brake (gasps of amazement! 
