Thank booba for the pic and Seattle FSB for his info.
Vacuum Leaks; can cause a lean,. condition, fast or rough idle, hesitation, stall, misfire and/or a Diagnostic Trouble Code (DTC) .
HVAC System leak "...The air handling system is designed to provide defrost when no vacuum is applied to any of the three air door vacuum motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak. Instead, a leak in the vacuum control circuit will send all airflow to the defroster outlets. This condition may occur during acceleration (slow leak), may exist at all times (large leak) and may happen only when certain specific functions are selected, indicating a leak in that portion of the circuit. ..." by Ford
"Intake Manifold Vacuum Test
Bring the engine (6007) to normal operating temperature. Connect Rotunda Vacuum/Pressure Tester 164-R0253 or equivalent to the intake manifold (9424). Run the engine at the specified idle speed.
The vacuum gauge should read between -51 and -74 kPa (15 and 22 in-Hg) depending upon the engine condition and the altitude at which the test is performed. SUBTRACT 5.5 kPa FOR EVERY 500 METERS ABOVE SEA LEVEL. SUBTRACT ONE INCH FROM THE SPECIFIED READING FOR EVERY 1,000 FEET OF ELEVATION ABOVE SEA LEVEL.
The reading should be quite steady. It may be necessary to adjust the gauge damper control (where used) if the needle is fluttering rapidly. Adjust damper until needle moves easily without excessive flutter.
Vacuum Gauge Readings, Interpretation
A careful study of the vacuum gauge reading while the engine is idling will help pinpoint trouble areas. Always conduct other appropriate tests before arriving at a final diagnostic decision. Remember that vacuum gauge readings must be interpreted with care.
Most vacuum gauges have a "normal" band indicated on the gauge face."
See and read more @ 1996 Bronco/F-Series
And See my Vacuum leak test in post #11 @ Help with dtc codes and idle