Recover any remaining r12. Flush out old mineral oil(hard to diy, but in a pinch unhooking everything and blowing out the components with compressed air will work) Change receiver/dryer. An expansion valve made for 134 will work better. Use PAG or POE oil(I'd say 100 weight based on the mineral oil it used) R134 charge is around 80% of what it held in r12. The sight glass will be useless, there are always air bubbles with 134. I put conversions under a vacuum for an hour before recharging. Get adapters and mark what oil and how much 134 it took for future reference. Don't forget to change any orings where stuff got unhooked.

Pressures will be slightly higher than with r12, and it isn't as efficient as a refrigerant so if your car doesn't cool properly with a good charge of 12, it isn't going to work very well.

I've done several on older 87+- Jettas. Be careful not to over fill and stall the compressor. Change the expansion valve at the firewall to a 134.

The high speed fan will run constantly due to a false reading or high pressures on on a hot day--99 or higher. Does it on all my retro's. I finally figured out its the high side valve that gets confused. ONLY on hot days.

Disconnect your fan motor connection momentarily

You want to put in LESS than what is required in the R-12 install

It leaks out faster, due to the smaller molecules

I put maybe 1 or 2 cans in to refresh the systems every summer

105 degree days are a *****, but once the car is up and running at high speeds the air is cold at the vents

My experience (before we kicked a tenant out of Canery Row warehouse who left 10 30# virgin R-12 containers full)