Expansion valves have different spring rates in the powerhead and different orifice sizes for different gasses. R12 is a larger molecule and runs at a lower pressure than R134a. As a result, R12 has a weaker spring and a larger orifice for the same capacity compared to R134a. You can run R134a on an R12 expansion valve, but you lose a fair bit of capacity in the system. If you run R12 on an R134a expansion valve you will run low suction pressure and can ice the coil (along with reduced system capacity).

It's a big reason why I constantly beat the dead horse of "always change the expansion valve" when redoing the A/C system or converting to R134a. It matters. R12 was banned by the Montreal Protocol in the early 1990s and you won't find service parts for R12 systems anymore. If you plan to run R12, you must run the original expansion valve, not a replacement which will be for R134a.

__________________
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]