The control valve is on the passenger side of the car (in North America...) somewhat near the differential. It is a silver colored contraption, perhaps 4 inches by 4 inches. There is an arm pointing to the left, which is connected via an adjustable rod and a second arm to the rear sway bar. When the rear suspension of the car extends or compresses, the sway bar rotates. The arm on the sway bar translates this rotation to linear (more or less...) up or down motion which moves the lever on the level control valve.

When the lever is moved above horizontal, the rear of the car rises. When the lever is moved below horizontal, the rear of the car drops.

You can test the suspension easily. Crawl under the rear of the car and disconnect the rod-to-the-sway-bar from the input lever of the control valve. A single 10mm nut is all that must be removed. Move the lever up, then go start the car. The rear end should rise - it takes about 2 minutes to extend all the way, the car should rise about 3 inches.

That said, I don't think misadjustment is your problem. The control valve always maintains a certain minimum pressure in the levelling system, no matter where the input lever is rotated. When empty, your car should be at the correct ride height at this minimum pressure. The only time the level control valve adds fluid (increasing the pressure) to the system is when you add a load to the car - either hauling sod from Home Depot, carting around your obese in-laws, or whatever.

So it sounds like you have no hydraulic pressure in the system back there. How is the fluid level in the underhood reservoir? How does the car ride - too firm, or too bouncy? Does the tandem pump (combination power steering and suspension pump in one) leak? Have the accumulators ever been replaced?

- JimY