1990 740 can't get warm air in to cabin.

[bvc]

I am familiar with volvos and have check many things and still can't get it figured out.I change environmental control panel because nothing worked. Now I have the blower motor working fine and the direction control working. My problem is that I have no heat coming out. The heater core is not broken and has warm or hot water passing through it.I know there is a damper to change from hot and cold air,but this car has a vacuum system that operates the dampers. There are 3 diaphram looking vacuum things and I don't know which one operates the damper I need.