I just relaying what has been posted before. Your problem is a common one with the mid 80's 700's. I went into the FAQ site and found some info.

Vacuum Check Valve Failures. [Query] I have a 1990 760T and the H/AC is losing vacuum during low vacuum and during boost from the turbo. There is a tee above the intake that has two black & white fitting that connect to two smaller hoses that go to the fire wall. I suspect these supply vaccum to the H/AC but does anyone know if these are actual check valves to prevent loose of vacuum or boost from entering the system? [Response: RL] There are numerous sources of potential vacuum leak (including some of the vacuum motors (bellows) on the actual heat AC system) but those two valves are a common problem. They are check valves to prevent lose of vacuum and if you bench-test them individually, you may find that they perform well under a fair amount of vacuum but that as soon as you try minimal vacuum, the valves will give up. You may try cleaning them out in WD40 or some rubber-softening substance but failing that, your best bet is probably an OEM replacement. [Tip: Chris de Courcy-Bower ] If the air conditioning misbehaves and hisses when accelerating, check the two "one-way" valves that connect the climate control system vacuum pipes to the inlet manifold. If they let ANY air through in the "from manifold" direction when you gently blow into them then replace them. (The good news is they are cheap.)