f johns- of course peter is giving you the ''right and true" solution above. my method is without using a vac pump. you put your finger over the line in the brake booster to see if there is vacuum.
if it is blocked the 'tee' can be drilled out carefully. dont go through the other side of the brake booster line!

to isolate the shutoff just hook up vacuum straight from the brake booster line to the green line going from the yellow valve near the firewall. that will eliminate any climate control functions but isolate the shutoff so you can test it.

good luck.

__________________
1984 300D Turbo - 231k....totalled 11/30/07 RIP