Sadly converting is Not as simple as most YT videos and other claim. Oh You often can do this but not under $50 to $100.
For one thing, Most R12 hoses and seals have problems after ~ 5 to 10 years let alone 30+ years.
Worse, Most old R12 and R134a compressors have bad seals/gaskets and not just main drive shaft seal buried under the clutch. GM V5 and other compressors are well known to leak thru case seems because "rust" on cases stops sealing them.
Either or more likely both are Why you have no R12 now.
If you leak R12 then you will leak R134a and faster because of size of R134 molecules. This is why R134 systems have "Barrier hoses" to prevent R134 going thru the hose material.
Many AC Shop Will Not work on R12 cars now and Many won't work on R12 cars converted to R134a either. Or worse R12 Systems loaded w/ Propane, "Freeze12" and other BS.
Why?Many shops do not have equipment to
Legally handle R12 now. Each refrigerate needs a complete set of recover/recharge machines, tanks, etc, to meet state and fed laws/rules.
Takes to much shop time to fix all the things wrong w/ R12 systems or system converting to R134.
Car AC shops Do Not have tools or Insurance to work on cars w/ Illegal refrigerants. EPA may approve X for their SNAP program but USDOT/NHTSA only allows R12 R134a and R1234yf but for only for cars made for R1234yf.
Most DIY "Conversion kits" Do Not have good service valves or no valves at all. Just very simple adapters to fit R134a gauges and tools.
Most DIY do not install even these crap kits right and worse contaminate the system w/ leak stop and other BS that can wreck recover machines.
Than add GM R12 high side valve is Not a simple Schrader valve and have issues right from the factory but old R12 hard valve covers stop or majorly reduce leaks there. Convert to R134 then often that valve leak bad unless you install an adapter w/ a primary valve in it that most Kits Do Not Have and even then they often leak.
TFS and others love to take your money but buying these "conversion kits" and car parts from them is not a good plan more so when they have a Core Charge.
AZ, Advance, and some others you can buy online and local stores take Cores, Returns and Warranty Returns so you don't pay shipping back to X.
Returns because new and rebuild units can be bad right out of the box.
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)
The Ogre's Fiero Cave