R134A is a wild material. The molecules of it are much smaller than R12. The stuff will find every little place to leak out it can. For cars that have been converted, it is even worse. The stuff runs right out thru R12 hoses. That is why R134A systems use "barrier hose". Also, the o'rings are different. Topping off the system with more R134A is fairly acceptable, but you should consider two things: If the system was R134A from the factory, or even if not, you should see if you can find the leak-probably with a dye charge. If it is leaking R134A, it is also leaking oil! Eventually it will run out of oil and lock up. At least add a little oil with the R134A, or better yet have all the leaks fixed and have it properly charged with gas and oil.