1998 Century A/C Question

I have a 1998 Century and I've always noticed when I would start the truck up and there was no air in the system and I would blast the A/C / heater the air would barely come out. As soon as the trucks air pressure reached a certain PSI like around 50 give or take I would hear a pop sort of noise and then the air would come blasting out. I have always assumed it was an air actuated door that went by the air pressure. Recently my blower motor went out and I replaced it. Ever since then it seems as though that "door" that popped open in the past seems to stay shut and the air barely comes out. It blows very cold but it just seems like that door isn't opening (if that's the problem). So, I was hoping someone would know what the problem is or if it IS that "door" where it would be located. Thank you for your time.

 

Thanks for the help. I actually went into a Freightliner dealer near me and they said they didn't have the diagrams for that. I found it quite strange but I will have to go to another one and aske them. I really appreciate the response. Thank you.

 

Thank you. I will see what I can do.