Anytime that you open the system, you must replace the drier canister (dessicant). Converting is not always straightforward. If the compressor is shot, there may be debris in the lines and expansion valve. Did it seize, or have bearing noise? If it seized, you really need to flush the entire system and re-oil it with ester oil - about 4-6 oz total. Ester oil is for conversions, PAG oil is for systems that were always R134a. Using PAG in an R12 system that is not flushed *completely* clean can cause clogs. Regardless, when it's all back together, you must draw a vacuum with a vacuum pump, down to 29 inches or better for at least an hour - longer is better. Once it holds vacuum, then you can refill with R134a, to about 85% of the R12 capacity. You will want gauges on both sides as well as a vent thermometer to assess how it's working when it's near full.