Heating Problems 88 5KSQ

[B1biker]

With the onset of cold weather here in Nebraska I'm having trouble getting the heater to work properly. I have changed the thermostat and checked the operation of the programmer, it seems to be working properly. The reicrculating flap is broken and in the 'fresh air' position but its been that way for several years and I have not had this problem in the past. My primary questions relates to the temperature control valve(s). The Bentley seems to indicate that there is only one temperature control valve in the system; this being the vacuum control valve located in the engine compartment. 
1 -- Is this valve open (full flow) position when vacuum is applied? (lever moves up and down freely in response to vacuum)
2 -- What is the likelihood that the valve is broken internally and is always partially closed even though it 'appears' to be working? 
3 -- Is there another valve that controls water flow to the heater core? If so where do I find it?

Any help will be greatly appreciated.
Gary L. Meier
82 CGT, 84 4KS, 88 5KSQ