NEW YORK (Reuters) -The Federal Reserve’s effort to shrink the size of its balance sheet is moving into a more uncertain phase as a key proxy of excess liquidity has been very nearly extinguished. Over recent days cash flowing into the central bank’s overnight reverse repo facility has fallen very close to zero, capping what has been a long and slow grind down from a peak of $2.6 trillion on the final trading day of 2022. The facility takes in cash primarily from money market funds, which use it to park liquid assets beyond what they need.