Bad accumulators cause a harsh, unsprung ride. The sagging is either a leak (which is obviously easy to spot) or an internal leak in the rear actuator valve. If you aren't loosing fluid it's the valve.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold