Vacuum leak

Hello all. I have a 2001 Explorer 4.0. The problem was a no start unless I held the accellerator almost to the floor. I had a friend crank the motor while I listened around the engine and heard a leak at the driver side fitting underneath the upper intake tube. I pushed on this fitting with my finger and it started right up. What is this fitting, and how can I repair the leak? It looks like a fuel fitting connection, and is connected to the passenger side tube. There is a tee in the line between them, and that line dissappears behind the accessory brackets at the front of the motor. It has started ever since I pushed on the fitting, however it revs up for a split second every time now. Also, the fuel mileage went from around 16 to around 10 mpg. Thanks in advance-Chris:smile: