I'd have to re-review '76 primary for my own benefit. As for '92, Brown was never going to get the nod, regardless. He was too strange and unpersonable. Prior to 2000, there was a much larger "moderate" contingent that was involved in the nomination process which is considerably smaller today. Blacks, despite being a huge % of their vote, had very little impact with the selection of Kerry. Even if Warner manages to get to the Convention with a plurality of the votes (which I sincerely doubt at this point), he's not going to get the nod. The only way that could happen is if the Democrat party all of a sudden adapted a pragmatic approach, and that's not going to happen within the next 2 years.
Brown is strange and unpersonable, but if he hadn't declared in the New York primary that he might choose Jesse Jackson as veep (which got him no Black support but cost him all his Jewish support) he might have won it. After his New York loss Brown was finished.
In 2004 it is true that the Black vote was marginal to the selection process. The Moveon types solidified around Dean early and Gephardt and Lieberman didn't get any traction (I wonder why Gephardt didn't solidify Black support ? Ideas ?).