If your system has hydrolic fluid in the reservoir, it is fairly clean, not dark or black, and the filter in the reservoir is in place and decent, I would say you need new accumulators in the rear. They are like shock absorbers in your car. Your car has what looks like shocks in the rear wheel wells, but they really just act as a hydrolic lift of sorts, the dampening action a normal shock would do is mainly accomplished by the accumulators. They look like large black softball sized orbs under the rear of the car and have hydrolic lines running to them. They do most of the dampening and the bounce control. When the seals in them go bad and the pressurized nitrogen gas in them is no longer pressurized, you get a crumby ride. I suspect they need replaced, it is common for these to go bad in time. They are an easy, but messy job and always use a flange wrench on the hydrolic lines casue they will round off very very easy. Be slow, deliberate and use a lot of penetrating fluid and they will break free with no issues. Get "strong man" disease and you will ruin the lines fast.

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red