Heater blows cool - what else on the checklist?

[DeadheadNM]

The underdash guts of my 72 Blazer with A/C are exposed as I added an am/fm radio recently and need to replace a leaking heater core. This winter I had lukewarm air blowing from the vents and want to fix this problem when I replace the heater core. So far here's what I've checked to trouble shoot the lack of hot air:

- coolant level is fine
- have hot coolant flowing to the heater core past the heater control valve
- vacuum lines seem routed appropriately with no apparent leak
- A/C control lines seem appropriately routed

I'm concerned about some broken/stuck flapper valve or bad vacuum actuator somewhere. Which of these duct work components should I inspect? Should there be a noticeable vacuum coming from the vac ball affixed to the A/C condenser box on the firewall with the K5 running?

I'd appreciate any and all suggestions.

Thanks, Pete