Converted my car to r134a gas Sanden compressor new evaporator and condenser hoses. Totally unimpressed. I was told I have to change the expansion valve as the 134 is differant but the expansion valve I received looks exactly the same and has no distinguishing marks. Question is how does the seller know what his selling,

 

Rex,

TRUST in my day in the Parts Industry, sadly a lost thing today.
Also the Part numbers are different based on supplier codes.

BUT

Down here, R12 spec parts have been unavailable since the late 90's, and DIY A/C work is outlawed.

The innards are the difference, mainly the orifice size.

ALSO,

Overfilling the system when using 134a gives what you state, so "sometimes" a SMALL bleed off of gas fixes the issue, but not always.

 

Rex
I have a sanden in mine and have done for years, and whatever the modern allowed fluid is, and it works a treat. You do need a new valve though, if your car still has the one from the old freon system. The modern fluid are at far greater pressures than fron was, and I think that is the reason the valve has to be changed.

 

Just for info/reference if it helps with expectations, I recently regassed my standard 1989 system with the original Harrison compressor & expansion valve, with R134a + Ester oil. It's the same setup as used on some 1970s/1980s Rolls Royce models & the specialists for those models seem to use the original system on R134a with Ester oil with no issues.
I was going to buy the Fenair kit with new Sanden compressor/expansion valve/hoses etc but thought i'd try it as it is.

I think the end amount of refrigerant used was around 950-1000g (Slightly less than the full R12 charge)

Only physical changes I made were a new condenser & receiver/drier (My car already had R134a ports from years ago when it was regassed with an R12 drop-in replacement 'RS24'). It gets cold & works fine (at least for now). Infrared temperature on the fuel cooler metal nut indicates 2-4 degrees C.

Edit: Here's the link to the 'kit' for the Rolls Royce - just charge ports & ester oil. If you go to 'additional information' tab the fitting instructions are in there:
https://www.flyingspares.com/r134a-c...134conkit.html

 

If you're using the RS24, which is a hydrocarbon, there is no need to change the oil or the expansion valve. Some expansion valves are adjustable, so that might be an option too.

Even within the R134 valves there are many different specifications for tonnage and the amount of superheat, it depends on what the OEM specified. There isn't a "one size fits all" valve.