I would suspect the proportioning valve on the IP or the vacuum transducer on the fender.

You may want to try using the MitiVac to control the vac on the modulator directly -- this will tell you if the modulator is working. Shifts should be smooth and sloppy at high vac and harsh at low vac, and you should be able to adjust them.

You can also bypass the vacuum transducer and connect the tranny directly to the proportioning valve temporarily to see what that does.

The rest of the vac circuit is for the 50C shift softening, and it shouldn't cause you low vac problems.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!