Jen,
 Go to Wal-mart, and in the automotive department, they have a 134 from
R-12 conversion kit, which consists of a oil that allows you to put 134
where the R-12 was, with no ill effects.
Evacuate the system, put this oil in, charge the system with 134----your
done.
No changing O rings, dryers, anything.

Simple, cheap.it works.

However, you best discover where the system is leaking down, prior to
doing anything.

Later,

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Terry 85 GL