Had a similar issue here.
I replaced both rear shosck and springs about 3 months ago with genuine GM shocks and KYB springs.
All seemed to be fine for a while but thne almost out of the blue the rear end seemed to be sitting too high.On checking measurements it was sitting at 705mm through wheel center to bottom of arch.
I did as suggested and removed the lower ball joint on the arm and with ignition on pushed lever to its full travel which raised the rear end to silly height.Then pushed arm down to lowest position and turned ignition off.I couldn,t hear any air escaping from the pump under the o/s front wing so assumed it wasn,t allowing stored air to escape and bring car back to correct "un-charged "level.Back under car and reconnected arm and turned on ignition again and pump again cut in to run but car din,t move...expect it was at its highest point.
Removed 1 of the air lines from shock and massive ecape of air and car went back to normal ??665mm level.Again ignition on and pump cut in but this time back end didn,t go up so assumed it had corrected its self.
This saga continued for a week or so until I decided to get a replacement pump and sensor unit to see if that was the problem.Fitted replacement pump and not only is it now noticeably quiter but when you turn off ignition you can hear the air escaping from the pump as I imagine it should.
To conclude it seems the pump has a self purge built in which allows stored air to be released when the ignition is turned off that way when you turn it back on again it brings car up to correct level.What seemed to have failed in mine was this causing more air to be continually be pumped in just increasing the height on every start and only allowing a very small ammount to be released.
I would suggest starting with the pump as the rubber air lines seem to perish and cause lots of false readings.I replaced all of mine on the replacment pump with 6mm internal diameter fuel pipe so all nice and air tight now.
Best of luck with it though.