Just a thought here,

Since I still have R12 on hand, I haven't converted a car to R134A yet. But, I thought that the high pressure switch on the drier/receiver had to be changed to one with a higher cut-off pressure, since the R134a operates at a higher pressure.
Is your compressor shutting off too soon, or is it operating and not cooling very well.
Also, it was my understanding that the system requires approx. 80% of R134a compared to R12. So, if the system is overcharged with R134a, this could be your problem.
Please post the fix to your problem.

Good luck,

Dave

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible