The vac supply for the climate control is the green line in the engine compartment. Best thing to do first is check for bad rubber connectors on all the vac lines -- I had to replace all of them on the 300 TE when I bought it, they were all rock hard or mushy.
After that, go get a Mitivac hand pump. You can attach it to the green line and pump -- this will apply proper vac to the AC system. If it won't pump down (there is a vac tank, it will take a while), you have a leak, and you will be able to hear it as the air sucks in if you keep pumping.
The fact that the system partially works indicates a leak rather than a defective PBC unit.
I'd guess a loose line, but could be the vac servo leaking, too.
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!