For the past few years, the AC in my 91 300TE (converted to r134 by PO) was only adequate at best, even with the correct r134 charge and pressures. So, I finally:

1. Pulled the dash and replaced all vac pods and rubber vacuum pieces.
2. Pulled the wiper and blower motor to clean the evaporator with HVAC coil cleaner.
3. Then I figured out the mono valve was not fully closing, so replaced that.
4. Had the r134 charge checked, and topped up.

Results, on original compressor and evaporator:
-July outside temps 95-102, center vent temps 58-62.
-August temps 80-90, center vent temps 48-52.

The 124 ACC system is complicated, and if just one component is off or bad, the system will not perform as we'd like.