I think that's a wise move. Even though R-134a appears to be on its way out in favor of R-1234yf, I would expect the former to still be available for quite a number of years due to the large number of vehicles on the road that still use it.
My references for your truck don't give specific refrigerant and oil capacities, only a note to see the under-hood decal for all three engines available that year. If you're lucky, the decal will still be there. Lacking the decal, figuring on 80-percent of the original charge guesstimated to be 48 ounces with the old York compressor is probably a good start. Fortunately, you already know what the oil spec is for the replacement Sanden compressor.rattle_snake wrote: ↑Fri May 28, 2021 8:23 am Since the only part left from original system is evap and thermal expansion valve, not sure what capacity it needs. Guess I will start with 80% of 3 lbs = 38 oz 134a and go from there. comp states 175 cc, so 6 oz, PAG46. Have new hoses and new drier. Plan to get new cond. Only have to flush evap/TXV. Assume I should inject solvent in high side.
Converting from R-12 to R-134a has been covered in the Forum Archives, as well as a "sticky" that Tim posted elsewhere. If you search the archives you should be able to find some additional pointers on the charging amount, what the pressures should be, vent temps, etc, rather than repeating all that here.
If you're going to remove the evaporator, I would flush it both backwards and forward (low side to high side, and then high side to low side). If you're leaving it installed, I would flush from the high side to the low side to ensure that you get out all the old oil and solvent.
Use care when cleaning the TXV, don't take it apart, and don't change the setting of the superheat spring. If you change that adjustment, you'll have to send it out for re-calibration or replace it with a new TXV.