After driving with bad accumulators the system often fails to lift anymore. This usually solves itself after the accumulators are replaced.
The idea behind the accumulators goes like this: The rear struts are given the job of supporting some of the load in co-operation with the springs. As load is applied the valve allows the pressure to rise the whatever is necessary to maintain level ride. At this point if the struts were full hydraulic and the car hit a bump the pressure would rise but have no where to go and the strut would act as a rigid membe. The accumulator is originally filled with high pressure Nitrogen pushing a diaphram against the wall of the sphere. As fuid pressure is built the diaphram is compressed till maybe half the sphere is nitrogen and half hydraulic fluid. The pressure of the Nitrogen has doupled and the total pressure now lifts the car. When a bump is hit the shock now can push the fluid into the remaining area of the sphere which compresses the nitrogen to very high pressure which then recoils like a spring.
Over time the Nitrogen is lost through the diaphram and the system becomes "water logged". Then the strut acts as a rigid member.
Bosch Master, ASE Master, L1
33 years MB technician