Did you replace that pressure switch with the same original equipment part designed for R12, or one designed for R134a? Most AC parts suppliers offer a lower pressure retrofit switch, and recommend using one due to the pressure differential between R12 and R134a systems.
You mentioned the problem occurred "in the process" of adding refrigerant. Did you eventually get the proper amount of R134a in, (you only need to add about 85% of the original capacity of an R12 system when retrofitting it to R134a), or is it still undercharged?
JS Auto Supply