I've got the same problem with my air as well. A year ago I spent $$ charging it (R12) and 2 weeks later it lost its charge...completely. Basically I need to replace all the seals. If you are upgrading to the R-134A, there are a few things to know if you want to do it the right way. First off, you will need a new dryer for your system. Any time you "open" the system, you should replace the dryer. And, of course, the above mentioned seals ('O' rings). You should also look into a remanufactured compressor. A lot of people may disagree with me on this, but to do it right, you should replace it. The reason is the R-134A runs at a higher compression than the old R-12. That higher compression can burn out your old compressor. I've heard of people who have upgraded and their compressor is still running fine, but I'm going to want cold every time I turn on the AC. If you have the $$, also go for the shorter condenser. It is a 3-core design and allows airflow directly to your intercooler. If I remember correctly they are around the $140 range. George Davenport (gldiii) had some info on them a few months ago. You can do a search for that. Assuming that your evaporator is ok, that is all you will need to get it going again. LOL! Personally, I am planning on putting all new items in to my AC system so I know that I'll have trouble free operation for several years to come. Which also explains why I haven't done it yet. It'll cost somewhere around $1k I'm estimating. But that is for everything new.
After thought...You won't need to put as much R-134A in the system as the old R-12. The new stuff operates much better with less in there. I'm not too sure how much less though. I've heard 80% of the R-12 volume, but am not too sure about those #'s. Check with an AC tech to be sure.
At the very least you'll need a new dryer, and to reseal your system. I'm not sure what that will cost, but I do know that it won't be near the $$ I'm planning on spending. Just keep in mind that you may have some pricey repairs in the future. I'm kicking myself right now for my demanding it be done right. So instead of some AC, I've got none in (the past week) 90*+ temps! And yes, my SC is my daily driver! It hasn't been too comfortable! You now have the way to do it that may cost you more, but will be less work in the long run. I'm sure that you will get the cheaper and easier fixes as well. I just think you should be informed of all the options before you decide how you're going to get it fixed. Hope this helped.