Last semester in college I had thermodynamics where my professor (whom has a doctorate in mechanical engineering) said that R-134a (which was supposed to be the cure for the ozone problems) is simply not living up to its previous expectations. As you probably know, R-134a was never as good as R-12 in an A/C system, but now it really looks absurd because it is just as bad if not worse on the ozone than the freon it was supposed to replace, R-12. Go figure.
And since I've enlightened you with one useless fact, here is another. The A/C system in your car is ~2X larger than the central A/C system in your house (rated in tonage).
The A/C in my 84 is very weak (all original), I think I'll add some more R-12 this summer. I shouldn't have to evacuate everything and start over, right?