The 134 Does not have the "Heat Transfer Rate" of the R12.
To successfully "Convert" an O.E. R12 W124 to 134:
1.Larger capacity CONDENSER A.K.A. "Parallel Flow".
2.Compressor DESIGNED and Constructed for the HIGHER Operating Pressures of 134.
3.134 TX valve
4.New 134 "O" rings (AND O-ring Lube)
5.New "Barrier" type Hoses. (The 134 [and it's lube] will bleed through the R12 hoses over time.)
6.Flush everything Except the New Compressor.
7.New Receiver/Dryer.

Without these as a Baseline:
All 134 W124 "Conversions" are Doomed to a shorted life span due to the Higher
pressures exerted on the OLD R12 components.

Then You're "Gambling with the Mafia" over the Evaporator surviving very long
with the excess 134 pressures,given their propensity for LEAKS with R12.
(Thank You Behr !)

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'84 300SD sold
124.128