only cold air blowing in my 1980 f250 with a/c

 

could be a bunch of things. stuck blend door. open thermostat. clogged heater core. bypassed heater core. extremely low coolant, to name a few things.

 

new theromstat, intake hose and return hose from heater core are hot, coolant topped off, vacuum hoses working and getting vacuum and opening and closing the vacuum valve under the hood near the fan motor. How can one determine of the the blend door is not operating?