Your SLS valve is leaking internally. Had the same problem on my wagon. The MBZ Classic Center has a rebuild kit for the valve. About $90. Be sure to index (mark) the position of the adjustment lever and rod before disassembly. There is a great DIY set of instructions somewhere on this forum. From memory - the system does not need to be bled once the valve has been rebuilt. The bleed screw is simply to relieve pressure on the system before removing the valve from the car. I rebuilt my valve about 1 year ago and the car has not dropped at all in that time. The rear springs seldom sag and the rear shocks seldom leak (according to Tom Hanson at the MBZ Classic Center).