Here's a good example of chasing the details-
Friend's 96 Honda Odyssey minivan, system pressures good, proper oil and
refrigerant charge, cooling fans working, etc., AC still not performing
well.  I just pulled the evaporator box apart a few hours ago and found
this:
https://lh4.googleusercontent.com/-9mr2bg1Ffmc/VVeYWy4p8oI/AAAAAAAAAaM/IlBF9
ueOE5I/w788-h591-no/SAM_1804.JPG

Vehicles built before cabin air filters came into existence (like the
Vanagon) are susceptible to this kind of fouling.