There is a lot of info online concerning the conversion. The most important one you are missing is a complete flush of the old system. A quick excerpt from one source is: A retrofit can be done one of two ways. The first is to follow the vehicle manufacturer recommended retrofit procedure. This generally involves removing all the old mineral oil from the system, replacing the accumulator or receiver/dryer with one that contains X-7 desiccant, replacing O-rings (if required), installing or replacing a high pressure cutout switch (which many shops seem to forget), changing the orifice tube or expansion valve (if required), then adding the specified PAG oil and recharging the system with R-134a. On some applications, installing a more efficient condenser may be recommended for improved cooling performance. After the modifications are made, the system is recharged to about 85 to 90% of its original capacity with R-134a.
I've done several conversions on older German cars and the systems never work as well as R12 unless a larger condensor is added; even then the results can be disappointing. You really need a vacuum pump (for evacuation and especially to check for leaks on your replaced components) plus an AC manifold set to do it right IMO or you are throwing good money after bad.