Not too good of an idea changing an old system, especially with an old ,
who know's in what kind of condition compressor over to the inefficient
134, that runs at higher pressure's , delivering 20% less cool air at
the ducts as the R-12.

Not a good idea at all.
Changing all the o-rings is good.
Changing the dryer is manditaory having an open system.
forget the rest.

Run either R-12 or an R-12 A, or an equivalent alternative refrigerant
that runs at lower operating pressures.

<<Don_Dixon@BELLSOUTH.NET
Michael,
You will need to replace the receiver/dryer assembly. You may wish to
consider changing all of the o-rings on the fittings you need to
break/attach with R-134a compatible ones, even if you do not change the
unit over to R-134a - that way, you can change it over later and
eliminate some future leaks. Personally, I would change it over to
R-134a now, since you need to refill the unit from the get go, but
that's just me...
Don in Reidsville, NC>>