The Federal Reserve’s decisions to keep interest rates near zero and to flood the financial system with credit are sparking fears of an eventual outbreak of inflation. But inside the Fed, an influential band of policy makers is fretting over the opposite: that the already-low rate of inflation is slowing further. The presidents of the New York and San Francisco regional Fed banks, William Dudley and Janet Yellen, see the abating inflation rate as convincing evidence the economy still is burdened by excess capacity and needs to be sustained by the Fed.