Check out this to your A/C questions.
http://dieselgiant.com/repairyourac.htm
Dont use 134a unless you want to replace the compressor very soon! Either use the original R-12 or Freeze 12. The pressures are much lower and move the oil through the system much better. The oil required with 134a is very hydroscopic and absorbs moisture like a sponge, very bad.