OK, I have had a similar problem before with a '88 16v from SA.

There is a slider that controls which vent the air comes out of that operates vacuum servos. It works by a rubber piece with holes in it sliding across and connecting different ports to open the various flaps.
The one that I had was not drilled correctly so that when a vacuum was applied to the rubber pad it closed off a minute piece of rubber and blocked the port. It took a while to find as at first we were blowing through it which opened it up. Then when I sucked on it I noticed that it closed up and blocked the vacuum.

There is a test procedure for it in the Bentley workshop manual.