Help-Trans fluid in the Shift Modulator vac. tube.

[DIYBenzer]

Need a little help on an trans problem on a 90 190 E 722.4 . I noticed ATF in the vacuum tube connected to the Shift Modulator. It's shuttering from 2nd to 3rd/ 25 to 36 mph. I have the replacement modulator and I know it must be set to 48psi for my trans. What I don't understand is how to pressure test the Modulator. My pressure/Vacuum tester only goes up to 10 psi and the Vacuum units are (in Hg) down to 30. What type of pressure gauge is used and where do you hook it up? How do you test the pressure to set the modulator and how do you set it when the pressure must be read at 31mph? If any one knows how to do this test procedure it would really help me out. My PM MB shop cd sends me to another section to perform testing, but that section isn't there.