Wee Robbie

I have replaced the accumulators and shocks on my 2001 4Matic Wagon, but the ride is still very jarring when passing over abrupt changes of level in the road. I checked the PS fluid two weeks ago and was surprised to see it lower than the end of the dipstick...I filled it up to the full mark. A few days ago I drove for 20 minutes and the PS fluid reservoir was overflowing! There is no external leak in the system....I've read that if the fluid goes down when there are no external leaks the fluid must be going into the accumulators. If this is correct what would cause the accumularors to later discharge the fluid into the reservoir?