Manifold gauge set: $60 Electronic Leak Detector: $100 for the cheapie, $200 nice Safety Goggles, Gloves, pocket thermometers: $25 License to buy your own refrigerant: $25 and a few hours of study.
Satisfaction of Doing It /Right/ Yourself: Priceless. ;-)
You don't have to change it over from R-12 to R-134a /IF/ the system is tight and leak-free - it's not illegal to keep refilling old equipment if it's tight, they just made it difficult and expensive.
And if you haven't had to touch it since the vehicle was new, it's tight - unless the compressor crank seal just gave out, holding more than 5 or 6 years between charges is pretty darned good. Just permeation leaks in the hoses and seals will make it go low after 10 years, 15 at the outside.
Your car is not all copper tubing and hermetically sealed like a home refrigerator, where tiny leaks are not an issue and 40 years on a charge is normal.
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