1981 300SD Climate Control R&R

[usmma2013]

Hello,
I'm planning to replace my leaking and worn compressor with a Sanden unit, and replace the TXV which is also leaking. While I'm at it, I think I have vacuum issues in the control unit but not sure where or how to start. Symptoms are slightly different than other threads I've read. The temperature knob functions. In all modes other than defrost, the front upper vents and rear center vent (behind console) will blow cold or hot to maintain whatever temp you select and it seems to maintain a temperature fairly well and keep fan speed to a minimum. In defrost mode, hot air comes out of the defrost vents only regardless of temperature selected (by design?) In all modes, no air ever comes out of the floor vents. Also, it seems to not allow the fan to come on at all (for heat or cool air) until you've driven for 10-15 minutes - long after the temp has passed the 40 degree mark. Again, not sure if this is by design but can't imagine I shouldn't be able to have cool fresh air blowing in immediately after starting.
EDIT: While I have the dash apart anyway, does anyone recommend replacing parts of this system as a matter of preventative maintenance? Vacuum pods, etc? I only want to have to do this once.
Thanks for any advice in advance!