1994 should have been factory R134a.

You really need to know what's going on with the high side as well. Without knowing what both high and low are doing, you're just shooting in the dark and guessing.

I'm going through the same thing in my SDL right now (different refrigerant). Center vent won't get blow ~65 in my case. Was working perfectly for a period of time, blowing in the upper 30s. I've tracked the issue in mine down to a stuck expansion valve. Stuck open in my case. Compressor pulled in, good charge of refrigerant, but low side is high and high side is low. Bang on the expansion valve and it'll work briefly until it moves again and gets stuck. You'd never know this was happening unless you had gauges hooked up to both high and low. You can watch it hunt a few times then "stick".

__________________
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]