Look for vacuum leaks around the vacuum relay in the air conditioning unit.
The vacuum is picked up from the engine bay and piped through the bulkhead
to the air conditioning unit usually near the reveiver/dryer mounting point.
It comes from the throtle block via two Nonreturn Valves. One valve is for
the cabin air temperature sensor (centre of roof, included in lamp housing)
the other valve feeds to the vacuum relay in the programmer and then is
connected across to the vacuum switch on the control panel. The Light Green
pipe connects the Lower Panel signal, the Brown pipe is for the Air Inlet
(Recirculation) signal and the Dark Green pipe is for the Upper Panel
signal. Light Brown pipe is for Lower Panel Shutter, Blue pipe is for the
Defroster Shutter, Orange pipe is for Air Inlet (Recirculation) Shutter, Red
pipe is for the Upper Panel (Vent) Shutter, Grey pipe is for the Water
Valve, Black pipe connects the relay and switch together. The last pipe
(Violet on 1983 & 1984) White (1985 on) is the source vacuum from the
bulkhead inlet. There are slight differences depending on whether Combined
Unit + Air Con , Automatic Air Conditioning (ACC) or Electronic Controlled
Conditioning (ECC) is fitted.

All the best, Peter.

700/900/90 Register Keeper,
Volvo Owners Club (UK).