Actually the valve assy could be fine. Usually,if a car drops overnight, or after a few hours, then the valve assy is allowing the pressure to bleed off, dropping the rear end. However, if there is a leak, as mentioned above, that could account for the pressure loss, and the sag-butt.
If it is rusty up under there, then maybe the arm isn't being moved by the torsion/sway bar thing, or even one of the sway bar links at the hub could be broken. Then the correct info on car height is not given to the valve assy via the link arm.
So, in my humble opinion, first the leaks needs to be found and corrected. At that point, then you can work your way to a solution, which is usually the accumulators.....
