I have a 1988 K5 Blazer, the question is, when the truck is not running I have plenty of air flow from the defroster, but when the engine is running there is hardly any air flow. I have checked all fuses, air ducts are clear, anyone know what I might be overlooking?

 

Sounds like one of the doors in the hvac system is closing off the air flow. The doors are vacuum operated and will return to a neutral position without vacuum.

 

I went outside to check and see if it made a difference if I was at a idle or revved up and it does not change.
I will check the vacuum tommorrow, I noticied a couple of line looked slightly cracked the other day.

 

Take a look at the attached vacuum diagram and system schematic. It'll give you an idea of what is supposed to have vacuum and what isn't for a particular mode.

The fact that it doesn't change from idle to wide open throttle makes since if the check valve in the system is doing its job. You likely have a leak in the system somewhere beyond the check valve, but it isn't enough to allow the vacuum to drop during quick WOT stabs.