Accumulators are a really good place to start. They will definitely help the ride quality. Though I'm guessing it won't help the sagging much. Unfortunately the SLS valve in your car is not rebuildable, like the earlier models. But the valves are the first place to look for leaks. You may find a leak that is allowing fluid to leak externally, but more likely the valve is leaking internally. The SLS and springs maintain the unloaded ride height. So if either is subpar you'll can get sagging. Does the rear lift when you load it down and the car is running? If so the pump is doing it's job. If not, you likely have a bad pump. The possible leaks spots are the rear struts, the connections of all the hard lines, and the pump. Make sure the reservoir is full as well.

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