134 is very different from R12. It operates at higher pressures so you need to be careful not to overfill. It is also not as efficient as R12. Use of a gauge set is preferable to the pressure gauge on those 134 cans. If the R12 has bled down completely then it's possible ambient air (with moisture) is in the system - meaning it should be vac'd out. Use of a compatible 134 oil is also important.

I went with "Eviro-Safe" and got great results. I also used the opportunity to replace all o-ring seals with a followup vacuum pump to expel all air and moisture and double check no leaks.