A rear that doesn't rise when loaded can be caused by two things, the suspention pump or the SLS valve. Does the rear settle or sag over night? If it's settling then the valve is probably leaking. Fix that then see what happens. If you aren't getting any sagging after sitting then your pump it the issue. You can get rebuild kits for the pump and for the valve from the stealership, atleast for the 123s anyway.

Might also check the lever arm while the car is sitting at normal ride height. It should be parallel to the ground. It not adjust it so it is. Then see if the rear rises when loaded.

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