You sure its the modulator? It could be the cap on the modulator. It is made of rubber and if the cap is broken, your modulator will not hold vacuum. Take out the cap and examine it. In any case, I dont think they sell the cap alone so you are probably going to end up having to get a new modulator. Get the new modulator from Mercedes since they have a part upgrade which includes a new pin. You can get instructions on how to install the modulator at the bottom of this page:

http://transmission.articles.mbz.org/adjust/

Mike Senko recommends setting the modulator at 41psi.(ballpark) and then adjusting according to the ride. Follow his guide at:

http://www.wagoneers.com/DieselBenz/TECH/Trans-ADJUSTMENT.html

You will need to make a modulator pressure tester. Here are some instructions on how to make one:

http://www.wagoneers.com/DieselBenz/TECH/Trans-Pressure-Gauge.html

Hope this helps,
Adiel