69 Barracuda heater control vacuum lines on firewall
My 70 dart also has 3 lines. Two to the heater valve and the top one to the intake manifold. My car is a slant 6 factory air. It doesn't have a vacuum reservoir. And I don't see where one would have gone. On my 73 camaro I did default to defrost when it lost vacuum. Everytime I went wide open throttle I would lose air through the vents until I let off the throttle. A simple little vacuum check valve fixed that problem. The dart does NOT default to anything. It simply stays where it was when I lost vacuum. So I haven't had the need to install a check valve.