I have a 1986 16 valve that also does the same exact thing. I notice the rear sags even after sitting over night. I always check the leveling fluid level and it doesnt change or drop so fluid leak isnt my question. Both struts are in good shape and the system was recently refilled with fresh fluid and bled by a M-Benz dealer. I think that the bladders may be going since the car rides pretty hard over rough roads. There are no fluid leaks around the stuts or the engine pump. Once I start the car in the morning, the stuts raise to there proper height. It is when the car sits over night that I suspect the system has some type of a fluid pressure loss somewhere.
Thanks, Joe