Seems overcharged to me. Both the high and low sides are reading higher than desired. Overcharging also reduces capacity; that would explain the cool but not cold situation. I would pull out refrigerant until the low side shows ~35PSI while running the engine at 2K RPM. Low side pressure is what controls the evaporator temperature.

In my experience Mercedes specs steep high side pressures. Still, 375 is darn high. You're probably close to triggering the high side cutout. While I'm not familiar with the 126 chassis, the 123 & 124 service manuals would state that ~300PSI on the high side is expected under those conditions.

So long as the high side is acceptable after setting the low side to the proper pressure I wouldn't sweat it. R-134a is going to run higher high side pressures than R-12. My gut feel is that anything below 325PSI under your stated conditions would be OK.

I see we are going to have 98 degree highs later this week; guess you'll get to try it out real soon.

- JimY