I converted my '69 Cuda Fastback to 134a. I prepped the system by:
- removing and cleaning the sump on the compressor to remove all the old oil
- changed all O rings to 134a compatible ones
- kept original hoses
- flushed the condenser and evaporator and all lines with A/C flushing fluid (removed the condenser and laid flat to get better flush)
- installed new 134a drier
- installed new expansion valve for 134a
- vacuum dried, added oil and charged with 134a to about 75% of R12 charge.
Works great!!!