I saw this post and I went back to an older one of mine and got this info : The procedure is to connect the gage hose through the tranny hump with the gage in the passenger compartment where you can watch whats happening as your driving. If your looking for a Tranny pressure tester for your MB I just purchased one from these guys,
They had one on E-Bay but I didn't want to wait so I called them at 724-224-5823 and talked to the owner Mike. He gave it to me at the minimum bid price $18 and shipped it right out. Its' a tranny/engine oil pressure test set that comes with a 300PSI gage, 6' hose and SAE and Metric adapters for $18 manufactured by K-D Tools for NAPA. I'm pretty happy with this tools especially at these prices so I thought I would pass this info along to anyone who might be needing something along these lines. Shipping for both tools was FedEx at $5.38. They included a listing of other tools they offer at very competative prices also. I have no interest in this just thought that I would share this wealth. These are the E-Bay listings if you want to see a picture or a description. These listings are old but they may still come up. Good luck!