A/C systems are very touchy and can be influenced by many little things, and you increase the amount of variables when you retrofit a system.
I did the same thing on my 89 Bonneville 5 years ago before I knew much about retrofits and a/c. I had a leak on the low side schrader valve also. I took out the valve stem and screwed an R-134a valve with valve core on top of where the old valve was. I used the cycling time of the compressor to get a feel for whether or not the system was full. It was still running great when I sold it in 2000.
Looking back, I skipped a bunch of steps like checking the high side, vent temps, cooling the condensor, vacuuming, replacement of the accumulator, etc. I feel lucky that I got it right!
Ester oil is important in R-134a retrofits. The existing mineral oil in the system will either all leak out or sit in low points in the system. It won't move around with the new R-134a. The Ester will.
You and I were fortunate, but I have heard more horror stories than successes with do-it-yourself retrofits.
1999 540iA Sport
Former 93 SSEi Owner
Former 89 SE Owner