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This has probably already been figured out, but here is a fix if someone searches this forum. 
 1977 280Z. 
Problem: Air will not come out of dash vents at any climate control lever position. 
 Cause: Mode door actuator not retracting to close heater plenum due to broken plastic boss at retract spring mount bracket on Heater plenum.
 Part location: Just above left hand side defrost outlet.
Applicable 1977 FSM pages: AC-31 Figure AC-41.
NOTE: The offending broken piece is NOT in front of the heater core or evaporator core. This repair is safe. However some common maintenance practice is required.
Non-standard Tools needed: Drill, Pop Rivet gun, applicable size drill bit. One applicable size aluminum pop rivet.
STEPS: 
1. Verify fault condition. 
2. Remove left hand seat. 
3. Gain access to the return spring bracket. 
4. Disengage the spring from the bracket and let it hang from the mode door actuator rod. 
5. Remove the bracket if necessary to finish removing the broken boss. 
6. Reinstall bracket and center the bracket boss hole in place where the broken boss was. (This will be your drill index.) 
7. Decision time. A 1/4” aluminum rivet repair this nicely. However, most hand rivet guns which will access this space only go to 3/16”. You may attempt using this or procure a gun which will handle a 1/4” rivet. I elected to procure a HF pneumatic Rivet gun. (Because I am a tool whore.) 
8. Using a drill matched to your chosen rivet size, AND using the bracket whole to center your drill bit, use low pressure high speed to drill the rivet hole. The plastic may be brittle. High pressure or low speed may cause your bit to rip a chunk out instead of creating a nice round hole.
 9. Rivet bracket to heater core housing. If using 1/4” rivet length of 1/2” should be fine. If using 3/16” use a washer on the head side to take up the excess space and go up a size in length to ensure there is enough “mushroom” on the backside. The good thing about the 3/16” is if it fails, you have a chance to go up to a 1/4” as a subsequent repair. As the rivet is in sheer, I don’t THINK it will fail if installed properly. Also remember to ensure you are pressing the bracket tightly against the housing when you squeeze the rivet, other wise you will end up with a loosely fitting bracket and cycles will eventually fail it.
10. Reinstall return spring. Verify the spring is in the indexes. The spring is surprisingly strong. Needle nose pliers are helpful.
11. Verify fix. 
12. Reinstall seat and done.
Hope this helps someone.
FYSA: The first pic is on my daily. The second two are on a heat/ac box I had laying around. I needed to verify the repair before proceeding on my daily.
Edit: I had my instrument panel under cover and center console removed for installing carpet, so I don’t know if it is required for this repair. (It sure was easy with them removed though.)

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Adding on to Sjoost's comment - the vacuum lines in the engine bay can split or break.  I had a problem with the small one that supplied the vacuum to the bottle from a port on the intake manifold.  It would split at the manifold, I'd lose vacuum and all I got was cold air from the heater.

 

Just filling out the potential causes.

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  • 3 weeks later...

I have an issue where my door gets caught on something when turning on the heat.  I can push on the actuator lever and it will, "pop" then open the rest of the way on it's own.  I checked it when I had the evaporator coil out.  Door seems to move freely when not connected to the actuator.  Not sure what's up with it.

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