Air suspension

Have a look at this, it's a technical service bulletin about what's probably happening to yours! I will email the complete bulletin, here's the partial copy.
Reference Number(s): 06-15-10, Â Date of Issue: Â August 7, 2006 FORD:2003-2006 Expedition LINCOLN:2003-2006 Navigator Related Ref Number(s): 06-15-10

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.
IF THE SENSOR ASSEMBLY NEEDS TO BE REPLACED FOR ANY DAMAGE/FAILURE OTHER THAN THE ONE DESCRIBED IN THIS TSB, THE REPLACEMENT ASSEMBLY IS SUPPLIED WITH A NEW STYLE LINKAGE ARM, SO THERE IS NO NEED TO ORDER ADDITIONAL LINKAGES FOR THE SERVICE REPAIR.

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AIR SUSPENSION PERFORMANCE - HEIGHT SENSOR LINKAGE ARM DISENGAGING
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 (Fig. 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).

One of the bags or lines are leaking enough to not let the car aise.

Have the system checked for leaks. You can use soapy water to find the leak yourself

Roy

Sure can.

Try this, open the door, close the door then start it up and let it sit, see if the pump comes on.

Roy