I was about to reply to your thread. I just been helping someone else with the 722.6...On another thread.

Now, yes it can be a vacuum. Put your car on ramps and unplug the vacuum line from the modulator. Start the car and feel the vacuum when you accelerate.. on the vacuum line. If oil is dripping from the vacuum line, the modulator must be replaced.. Feel the modulator if it is not loose....

Here are some documents that may help, but find master.pdf, and look at the troubleshooting section-See the first link...of post2

1996 S280 Australia transmission no reverse diagnose & rebuild. - Mercedes-Benz Forum

All the best,

Martin