r134a ?

It is always better to find and fix the leak. It's all the same priniciple. Leak stoppers will find very small places to seal. They cannot disseminate between the actual leak and the small holes of a part like a filter.

I am guessing that you have a 94/95 car. More than likely your leak is at one of the connections. An o-ring kit is less than $10. You could replace all of them. Then vacuum the system and recharge it and not have to worry about gunking up your system with some stopleak.

You could also take it to a shop that uses nitrogen and an electronic detector. These things are very accurate when used properly to find even the smallest of leaks. Again, I will lay money it's an o-ring connection.

Just me, but I like fixing something rather than bandaiding it.

i'm a refrigeration tech and I wouldn't put that stuff in my car...like the other guy said more than likely a o ring thats rotted out and leaking. not sure were you are at or if you have autozones by you...but down here they actually rent a vacuum pump and gauges out for free....you just buy some gas and pull a vacuum and drop in the exact amount thats on the sticker under the hood and enjoy some ice cold a/c. Usaully if the fittings are oily and aren't anywere near the engine...like up front by the condensor and by the accumlator then thats were your leak is....electronic leak detectors are nice but soap bubbles will do the trick or a good eye for oily fittings. hope that helps!