"
Until 1995, most cars with air conditioning used R-12 Freon refrigerant that is said to have a negative side effect of damaging the ozone layer and is no longer produced in most countries. Now, the industry standard is R-134a, which is an efficient absorber and carrier of heat without the ozone layer damaging issues of R-12. Contrary to popular belief, most cars originally equipped with R-12 can be converted to 134a and still keep you just as cool.
Recommended steps to converting your Ford R12 air conditioning system to 134a.
1962 & Newer Systems
Minimum Requirements:
- Change or restore the filter-drier.
- Drain the compressor oil and recharge with 134a compatible oil. (PAG or Ester Oil)
- Insure that the system is clean and free of contaminants (Depending on component condition, this may be a simple matter of blowing out with compressed air or liquid flushing with an air-conditioning flushing agent)
- Replace or restore all rubber hoses with barrier hose.
- Remove EPR valve from compressor’s suction port
- Install thermostat.
- Charge with 134a between 70-80% of the original R12 charge amount.
- Replace any o-rings with 134a compatible.
HELPFUL HINTS
- Ignore the sight-glass. A properly charged 134a system will likely show bubbles in the sight-glass.
- Typical 134a charge rate will be approximately 70-80% of the factory R12 charge amount. "
from
http://www.classicautoair.com/OEM-Air-Conditioning.html
not endorsing the company but it's a start
good luck