+1 to the full R134a retrofit.
The proper way to do it is to have the system drained, then pull it apart and put in new o-rings, and a new accumulator / receiver / drier. You can flush each part individually too, except the compressor. Also look very closely at the hoses. Any that have dirty grease like goo near the connections or where the rubber meets the metal, should be replaced because they're leaking. Also there's a different expansion tube you can get for it that will work better with R134a. It's not critical, but it helps. Put, IIRC, 2oz oil in each part, using ester oil. Then pull a vacuum on the system. The longer, the better. I only did 1 hour, but 1 day would be best. After all that, charge with R134a and enjoy your AC system.
If anyone wants to refine or correct my procedure, feel free. It's been a while.
The cheap-o retrofit might get you by for a while, but it won't work as good and you will end up right back where you're at now. I say do it right the first time.