He emailed the below information in response. Good stuff... thought it should go on the public record (with his permission of course).
Keith Bowers (Texasoil) wrote:The pressure in each strut is 'independently' controlled --as determined by the ride height sensor and the computer algorithms which take into account g-sensor inputs, throttle position and velocity,vehicle speed, transmission gear,etc.
Two of the pressure control valves are co-located in the Front control valve assembly (right front ahead of fender well liner, and two in the Rear pressure control valve assembly (located right rear beside the trunk well, under the plastic shield).
The system CAn become 'unstable' with flat main accumulators or wear in the individual valve spools.
Wear in the spool valves is caused by not changing the fluid and also running with flat strut accumulators.
The symptoms are 'hunting' for a height all the time. What happens is the electrical solenoids that push on the valve spools cannot exert enough force to control the strut pressure when lightly loaded.There is an adjustable balance spring for each valve.
Later model valves incorporate several 'advances?' that change certain characteristics, primarily noises and the sometimes abrupt 'jump' on shutdown. Fundamental control is unchanged.