How long ago was the conversion? I imagine someone who gave the shop carte blanche would have been happy with the conversion or done something about it. If so, have the system leak tested, repaired as necessary then charged with R134a. Consider an R12 “upgrade” if iR134a is incapable of a 115*F day. Consider also that the incapability of an R134a charge might not be caused by your refrigerant choice. It could be a tired compressor, clogged refrigerant line, etc.

Sixto
98 E320s sedan and wagon