Yes, sounds like you have no vacuum line to the heater controls. There are two vacuum ports on the 403 intake, one at the front for emissions, one at the rear behind the carb that runs to the heater controls. It will also have a one-way check-valve on it that can also fail or leak but you'd at least have something even if it was faulty,would just drop to other settings and open air doors when vacuum dropped too much.