The brief
Miss Elizaveta's 750i had sunk at the rear, sitting low at the back, so she brought it in to have the rear suspension checked. A saloon that sags at one end usually means the air suspension there has lost pressure, and a leak that's left alone only gets worse and overworks the compressor. The rear of this car rides on air struts kept inflated by an air pump. The rubber bag in the strut flexes with every bump, and over the years it perishes and starts leaking air, so the rear sags, the compressor runs constantly trying to keep up, and eventually it can't. A perished air strut doesn't reseal, so it gets replaced, and with the pump having been worked hard, that gets renewed too, the system bled and the ride height recalibrated.
The diagnosis
The check found the rear air strut worn, leaking air so it couldn't hold pressure, which is why the back had sunk. The compressor had been overworked keeping up with the leak and was due alongside. The rest of the suspension was sound. That's a rear air strut and air pump replacement with the system bled and the ride height calibrated, rather than chasing air leaks in perished rubber.
The work
The worn rear air strut and the air pump were removed and new genuine BMW-spec units fitted, every fastener torqued to the manual figures. The air system was bled, the new strut inflated and the ride height calibrated with the diagnostic system so the car sits level at the right height, and any suspension faults cleared. A road test confirmed the rear sitting level, the compressor cycling normally instead of running constantly, no warning light, and a composed ride.
The outcome
The rear sitting level at the right height, the compressor cycling normally, no air suspension warning, and a smooth, settled ride. Miss Elizaveta got the 750i back sitting right again. A perished air strut only gets worse and overworks the pump, so replacing both and recalibrating the ride height reset the rear suspension properly.