Heres how I converted my 89 a/c system from r-12 to r-134a.

Evacuated R-12 and oil.
Added adapter fittings to R-12 lines.
Pulled system into a deep vacuum.
Added oil and r-134a to system(4.24lbs)
Ate a piece of pie while charging....
Blows cold baby!

I work in a shop so this costed merely $20.

Works F'ing Fine!