B52bombardier1

Your old 1980 AC refrigerant is called "R12" and is of a vastly different chemistry than today's "R134A" and "R1234yf" refrigerants.  Their lubricant oils to keep the compressor from destroying itself are also incompatible.  Your "pilot operated absolute valve" or your "expansion valve" (not sure which you have installed) for R12 are not calibrated for best performance with today's R134A and R1234yf refrigerants.
  A thorough, expensive and complicated swap over is required to make it happen. Hoses, oil rinse, refrigerant fittings and a change to a more efficient parallel flow condenser are either required or highly recommended. It will damage the system if not done correctly. There are plenty of You Tube videos that cover how this should be done - as always with YT, some are better than others.  You might even consider simply repairing the leaks, repairing the now likely clogged expansion / POA valve and replacing the receiver-drier in your current R12 system - the R12 is not that expensive anymore. Avoid converting to the R1234yf stuff because it is extremely expensive.
https://www.youtube.com/results?search_query=r12+to+r134a+conversion+chevy
Rick