Have you completely flushed the system including emptying the struts? They may be acting as a reservoir for pieces of the membrane which may quickly plug the pressure holding valve/return orifices in the SLS valve. It is hard to imagine that pieces of the membrane could block the return line but I suppose it is possible. Have you tried disconnecting the return line at the SLS valve and back-flushing the return line? I recently replaced the accumulators on my 2001 E320T and was surprised to see pieces of membrane flow out of my left accumulator. I had not had this happen before and would not have believed it if I had not seen it. Since this was the case, I supported the the rear control arm, disconnected the strut from the control arm and pumped the strut several times to (hopefully) remove any impurities. No problems so far.

It is amazing how good the system is when it is working properly and how it slowly deteriorates over time until you finally conclude that the accumulators need replacement. Truth is, they probably needed replacement 20K miles before (my car has 115K miles).