2000 S500 rear suspension - pass side rear strut drops to stops intermittantly
 

Need some help from more experienced owners or a mechanic. Wife was driving home from work when the suspension too low warning lite came on. She wisely pulled over and called me. I went to pick her up and found the pass side rear suspension down to the stops. I started the car and could hear the pump run, but no improvement. Had the vehicle towed home and when I drove it into the garage, the pump kicked on and the light went off. I checked and the passenger rear was near normal. BUT, as the pump raised that side, it went up a little higher, and then I could hear air escaping from the wheel well. It stopped hissing as the suspension reached just below normal level. This morning it was about where it was last evening, so I jacked up the vehicle with the jack under the lower suspension arm & pulled the wheel. I figured I could locate the source of the air leak. After the tire was off, I lowered the jack to achieve the same ride height as before (about 25 1/2 inches above ground), but the pump would not go on. I replaced the tire, lowered the vehicle, turned on the ignition and the pump came on the same cycle of raising the pass rear side followed by a hissing leak as it came to rest back at 25 1/2 inches above ground. For reference, the driver side is about 3/4 inch higher. I am confused. If it was a failed strut, I would think that the air loss would continue until the suspension was back down to the stops. I am guessing that the strut has a relief valve to bleed off pressure when lowering ride height and that is what I am hearing, but don't know for sure. Can any of you gurus help with the diagnosis as I don't want to buy a new rear strut only to find that it is not the problem.