Volvo Repair: 740 volvo ac air ducks, vent system, reserve tank

Question
1991 740 volvo 4 cyl.  as you are getting on to the highway or under hard acceleration vacuum goes away and the dash ac air ducks close and go to defrost and heat with the ac air blowing out them but then as you back off hard acceleration the vacuum is back and air blows back out of dash ac vents  the car was low miles on it 30,000  it was running bad no power someone put timing belt on it and missed it 1 tooth I reset cam timing and engine power back check the vacuum reserve tank under dash no leaks hose to it had good vacuum what should I look for next  

Answer
there could be a leak in a diaphragm for the vent system or a bad check valve for the supply side of the vacuum.I have not seen many typical problems with the vent system on these cars so I can not give a best guess.all the vaccum system for the vents will need to be checked.