I don't think there was an official break-up episode. In fact, in the book Street Gang, it says that Northern Calloway was upset that they didn't have Maria marry Luis, but the producers or whoever thought it'd be risky to have an interracial marriage on the show, especially since they already went through a bit of trouble to get the marriage approved of in the first place. But then in the 40th anniversary book, it says that they originally DID consider having Maria and David marry instead, only for Northern to ask them to reconsider due to his poor health.
Considering Northern died a few years after the marriage, it probably is best that Maria didn't marry David. Not sure whether they would have wanted to an episode dealing with David's death (though it could have led to a possibility of adding single-parenting to the curriculum). Though I do wonder, when either of the actors playing a married character dies, I wonder if the absense will be explained.