Before switching to R134, investigate the alternative R-12. There is a great product R406 which can be used in place of R-12. The inventor george is very helpful and will discuss any and all topics pertaining to air conditioning. I took the EPA MVAC registration and then purchased some R406 (trademark name AUTOFROST). It is fantastic. It is a combination of R22 and R142b, and, as such it has the characteristics which approximate r12, only they run a little cooler (i.e. condenser is about 3-10 degrees cooler). The problem with 134a is that the oil is fully incompatible and absorbs water easily. This leads to large system failures generally a year or two after switching. R-406 does not have these problems. Just a suggestion, check it out
http://www.autofrost.com