I retrofit systems from r-12 to 134a . A little advice
1. Check compressor pressures, if you have pressures that fluctuate , replace compressor with new not rebuilt.
2. If compressor is ok , you should know that converting your system ,the air will not be as cold as r-12. It usually cools 15-20% warmer at idle.
3. Replace reciever drier and flush system. Do not vent your r-12 (the fine is very hefty). And yes fix the leak .
4 . I dont replace all the o-rings I have never had a problem with leaks but it doesn't hurt to run a leak test afterwards.
5. Now for system capacity, dont go by sight glass. Your system usually holds 2.4 to 2.6 of r-12. Now you want roughly 15% less than that so start at 2.0 lbs. Add while watching your pressures. I like to add in the heat of the day when it is hottest. You want your pressure yo be 35-38 low side (blue side) at idle. High side start off around 180-200 . As high side increases low side will also a little but when auxillary fans kick on pressures will drop.
But to let you know just like I tell my customers these compressors ( nipindenzo) dont cool well at idle with 12 in so dont expect it to cool like a new mercedes. Now the R-4 and a-6 compressors cool good . The only ones I recommend not to retrofit to 134 are 107 and 124's . Its a 50/50 on those cars .
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck