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The problem with your Navigator is likely a height sensor malfunction.The most common cause of this problem is a disconnected sensor link under the rear between the sensor and control arm.
There are no air bleeds in the system to manually drain the air out,the computer has to operate the vent solenoid on the compressor and electrically open the air spring solenoid and the related gate valves.
If the sensor rods appear to be connected properly,you will need to have the air suspension control module tested for falut codes to determine the root cause of the rear height issue.
There is a ford service bulletin describing this problem and how to fix it.
Some 2003-2006 Expedition and Navigator vehicles equipped with an air suspension may exhibit an air suspension height sensor linkage arm disengaging from the suspension ball stud or height sensor rotating arm. This concern typically occurs with the rear sensor (Figure 1) but may occur with the front sensors as well. If this occurs, the symptom will typically be the vehicle leaning to one side, the vehicle sitting too low/high, a harsh ride, and/or an inoperative air suspension. In addition, this concern may result in diagnostic trouble code (DTC) C1725 (air suspension front pneumatic failure) or C1726 (air suspension rear pneumatic failure).
When a linkage arm becomes disengaged, do not re-attach the arm and do not replace the sensor unless broken. Instead, install a revised linkage arm on all three (3) sensor assemblies, even if they have not detached from the ball stud. Refer to the following Service Procedure.
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