dont listen to anything anyone says about r134 in an r12 system. just vent whatever is in there and refill with r134. it would be even better if you could replace your reciever dryer and vac the system down. also get a can of the leak dectector stuff and squirt it in as well. they normally come with a uv light and you can run it for a day and then check for leaks with the light. i promise you no matter what anyone says the two freons wont hurt each other. again i promise you they wont. its just old wives tales. im sure someone will chime in and say im a moron and all that because some people on here are just like that.
john