There is a vacuum "switch" (well not really a switch but something) as part of the heater controls which determines, based on switch position, where vacuum is directed. It can over time leak and not hold vacuum and is nearly impossible to fix or find a replacement for. For a 63 vehicle I would start by taking the line from the carb to that switch off and check to see if it holds vacuum. If it does, then replace it on the switch and remove the air door and defroster door vacuum lines from their respective diaphragms and plug them. Then check and see if the system holds vacuum. If it does not hold vacuum, then check each vacuum hose for leaks. There may be some as it is an old car. If they are bad replace all vacuum hoses. If they are good, then likely the problem is the vacuum switch. If that switch is bad call Vintage Air or one of the others and replace the whole system as you will not likely find a replacement for the switch.
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