You are missing a few critical pieces of info. Ambient temp? RPM? Vent temp(s)? You need to quantify "all but disappointing."

Looking at my log, my W126 380SE pressures were 27/325 @ 2500 RPM 98 degrees ambient. I don't know what your ambient is, but the pressures seem a little low to me; they read more like R12 pressures, except the low side is too low (needs to be near 30). My log shows I used 3 cans of 12 oz. R134a, for a total of 36 oz. IIRC, this is about 75 to 80% of actual system capacity. My vent temp readings a few days later were 62 idling, 48 moving, 86 degrees ambient.

Keep in mind that the system was designed for R12. Using anything other than R12 in it will produce less than optimum results.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280