Eric:

Pressure control or vent back into the turbo intake? Don't know about the first, but there is a valve called the air recirc valve that dumps boost back into the turbo intake under computer control.

Sounds like your cold softening circuit is working, but the normal shift control still has a leak.

Keep looking, you will find it!

Also, since you have dead transducers, check the restrition in the hose to the big "T" -- it may have crud in it, decreasing the vacuum capacity.

Peter

__________________
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!