These accumulators use technology that was used by Citroen decades ago. The hydro-pneumatic suspension system is so good that Mercedes and Rolls-Royce used it. Inside the sphere there is a rubber diaphragm and above the diaphragm there is Nitrogen gas which is in fact the spring! Unfortunately after many years Nitrogen diffuses through the rubber and the result is that the gas-spring becomes harder and harder. In extreme cases such as what happened to a friends Citroen BX, sitting in the back of the car gets to be like being kicked by a horse! I told him what the fault was but he didn't believe me and in the end he got a garage to fix it. "It were them balls!" he said! Anyhow changing the accumulators will usually make the ride as good as new so go for it!
Wasted £500 on new rear struts, following wrong advice from my local garage, only to find that it was the rear spheres that were at fault! They had deteriorated internally, and had become hydraulically locked.causing a pipe to burst. To replace two hydraulic pipes, £168-78, (from Germany), and £220 labour charge, + the dreaded VAT, a further £185 for the spheres and about £100 for hydraulic oil. WOW. Oh, and another £105 labour to fit the struts and spheres!!! The struts are NOT shock absorbers!