Yes pressure reservoir is the item, the hydraulic struts are usually bullet proof unless they leak. The reservoirs are a spheres with a diaphram, one side is connected to the hydraulic oil side under pressure, the other filled with nitrogen. As the diaphram ruptures the gas escapes and as the spheres fill with oil it cant be compressed, in the early stages it is bouncy, it becomes hard as the system fills.
The good news the spheres are cheap, the bad news is the job is messy. The system works like this, oil is sent from a reservoir to a valve attached to the rear axle, and controlled via lever from the torshion bar. Oil in, rear rises, oil out it settles, the oil is constantly circulated.