Months ago, my 1987 300SDL abruptly began to run through the gears too quickly when accelerating to speed. I found the transmission modulator valve cap to have a vacuum leak, so replaced the whole valve, adjusting the pressure according to Mercedes specifications for my car and valve. The car continued to run through the gears too rapidly, and began to flare between 3rd and 4th gears, particularly when slowing down (foot off of the accelerator). The only positive change experienced was that the flaps internal to the air system inside the car worked much better.
I found that the vacuum control valve (mounted on the side of the IP) was not bleeding enough air, so replaced it. No help. The vacuum amplifier had an internal leak, so changed it. No help. Replaced all vacuum hoses (checked routing of connections) that even appeared to be getting old. No change. Changed the two switchover over valves on general principles, cleaned all openings to the various manifolds, etc. During all of this, used a Mity-Vac to check vacuum pressure, and currently the vacuum pressure appears to be working properly, except the vacuum to the vacuum modulator valve on the transmission drops very slowly to only 2 pounds of vacuum at full throttle at normal temperature. Adjusting the Bowden cable during all of this has had absolutely no effect on shift points, and the electric button under the throttle pedal doesn't cause a downshift.
As an experiment, I've adjusting the modulator valve one turn to the right (more pressure) to see if that helps with the flaring or shift points. No noticeable difference.
Also, with the vacuum disconnected from the modulator, no flare, but still shifts upward too quickly, and still no "kick down" shifting.
The soft shifting makes me think the vacuum level to the transmission is generally too high. My next step is to replace the 50 degree sensor on the thermostat housing the drives the switchover valve (new) that passes boost pressure to the vacuum amplifier, and checking that this circuit is operating the switchover valve properly.
Any ideas or suggestions are welcome. I'm wondering at what point to take it to a transmission specialist, but I'm also wondering who would be qualified to work on it in my area (Richmond, VA). For example, our local MB dealership doesn't do transmissions!
John

The early shifts and no kick down with the Bowden cable adjusted all the way points to a valve body problem, what is your working pressure reading in drive at an idle; it should be 75 to 90psi hot at idle and 160 to 195PSI at stall, if you have the pressure then the pump and modulator are working. The soft shifts would be valve body related. Steve Brotherton has an excellent article on the vacuum system

At 208k miles, a slow shift into reverse, and with a valve body problem, I may be looking at a rebuild as an optimal solution. Do you agree?
Also, I am very familiar with Steve's articles, they've come in very handy!
John

Just checking: any opinions on my previous message above?
John