I would say the t in the middle diverts flow to the main heater to the left and the rear heat to the right. That valve you don't have opens to allow flow to the rear heater when you turn it on. If it isn't there then water flows all the time. Like I mentioned, plug the vacuum line that would normally go to that valve and see if it changes anything.

 

Problem is that makes no sense. The bit that I don't have is where the grey vacuum lines connects. The other end of that vacuum line connects to the actuator for the face/foot vents (see below pic where grey and black lines connect). If I manually operate the actuator, then air comes out of the face vents (when I release, it returns back to foot vents). I can't see that it has any bearing on the air to the rear cabin. My guess is that flow in that main line causes a vacuum to be generated (through the 'missing' vacuum diaphragm) which can then be used to cause the actuator to operate and apply air to face vents via the grey vacuum line. Since I don't have any vacuum in that line, there is nothing to apply vacuum to the actuator

 

With the line open it is essentially a vacuum leak when vacuum is supplied to that line and it could disrupt normal function somewhere else. I don't see why you are reluctant to plug it and see what change if any occurs. It's not a permanent change.

The valve that the vacuum line connects to lets coolant flow to the rear, not air.

 

I've tried plugging it. Nothing happens.

 

That vacuum line supplies vacuum to the valve to allow coolant flow to the rear which it looks like the actuator determines that. The line does not supply vacuum to the actuator as that supply has to come from the engine and through your hvac control. The coolant valve is a dead end and can not supply vacuum.

 

so, maybe I don't need to worry about that line at all since the rear air doesn't matter to me. I made a video to help explain what Im seeing though


With the engine running, as I move the air distribution control, you can hear what sounds like the AC unit switching on and off (will need a recharge I know that much, since the air isn't exactly cold!!)
When I put the vent control to feet, I get air from the floor vents. When I move the control to face, you can see the actuator attempting to move...moves about 1/4". If I give it some assistance, it will push air through the face vents, and it will stay there until I either move the control to foot, or turn off the engine (in which case the actuator returns back to the foot position).
This is where I got my original thought that without that extra grey vacuum line, there wasn't enough vacuum to fully trigger the actuator

 

Mine is a 96 so things are different between the 2. Mine has been jury rigged and I haven't really delved into figuring stuff out. I will say that your air may be working fine, but it is having to overcome the heat from the hot coolant being circulated. Mine has a manual valve in the return line from the heater core that I hadn't really noticed as it was behind the other valve and coolant supply line. My a/c was marginal at best and when running with just the vents going with outside air it was hot. I found that manual valve and flipped it and now get cooler air through the vents and the a/c works as expected. From what I have read in the manual the stock valve is supposed to cut off coolant flow to the heater core when on max a/c or defrost. In my case that wasn't enough or it was leaking and allowing hot coolant to flow to the heater core which offset alot of the cool air from the a/c.

 

I appreciate your responses, thanks for giving your advice.
The hot/cold air is something I will have to tackle later, once I figure out how to get the air to come out of the right vents to start with. When operating the temperature control I can see the actuator moving, and on hot, the air definitely comes out hot!! More valves might be at play and I will have to poke around and see if I can figure that one out after the air direction issue is solved.

So many hoses going all over the place and my lack of knowledge is the biggest blocker here. I need to do a bit more research to figure out if I have a vacuum leak that is causing the lack of actuator action. Or even if it's a control **** issue (I might be able to do a hose swap on the back of the switch to see if the actuator works with a stronger vacuum). It looks like it wants to work, just isn't.

I also want to understand a little more about what clicks on/off when I turn the air vent control ****. I would expect to hear it when switching from Max A/C and face A/C to any of the other positions, I just haven't spent much time trying that one yet to see what it actually is

 

thinking about this last night. There are three vacuum lines coming from the air vent position actuator (red, grey and black as seen in my pic). Pretty sure (need to really get under there and check) the black and red lines go to the air vent control ****. So when turned to 'face' the vacuum should be applied via the black line and pull the actuator to direct air to the face vents. When moved to feet, vacuum changes the actuator to feet. The grey line (the 'problem' one), would normally go to that valve that I don't have. Would it make sense that the valve I don't have should shut off coolant flow to the heater core. Since I don't have this valve, the air is always going to be hot (or hotter than cold at least) because I'm not able to shut off coolant flow?

 

What I don't understand is why the black and grey line are y'd together right before the actuator. With those 2 tied together when vacuum is applied to the black line there isn't enough to pull the diaphragm because to me the vacuum is leaking out the grey line it is connected to. It puzzles me why when you plug it there isn't enough vacuum to pull the diaphragm. May be not enough vacuum supplied from the control ****. If you put a gauge on the black line and checked the amount of vacuum there compared to the red line. Is the default position on the actuator to the feet?