Just a day after former Treasury Secretary Robert Rubin resigned from Citigroup, the ailing financial giant is in discussions to form a joint venture with Morgan Stanley that would combine the brokerage units of the two banks and give Morgan Stanley 51 percent ownership. Morgan would pay $3 billion for its stake in the new venture, increasing its presence in the financial services industry and giving Citi a much needed cash infusion. “There’s been a lot of pressure for Citi to monetize some of their more valuable assets and Smith Barney is certainly one,” said one money manager. “There’s also been a lot of pressure for Morgan Stanley to look at how they can better lever their business units.” The combined brokerage would be the world's largest, employing about 22,000 advisers, compared to 20,000 at Bank of America after its purchase of Merril Lynch. Announcement of the deal could come as soon as today.