If R12a is supposed to be such good stuff, why has 134a been the standard for as long as it has?

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Suppose you were an idiot, and suppose you were a member of Congress; but I repeat myself. - Twain

Because 134a is what's been forced down our throats after R12 was taken off the market. R152a is not marketed as a refrigerant, which is why it isn't as well known as R152a or R290a (propane). Keep your shirt on, R134a isn't long for this world either.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]