Check what the low side is reading at around 1100 rpm. In cars where they can over time develop fuel injector issues that influence idle speed, the drop in idle RPM that can happen over a few years will sometimes give you a false reading on the low side. Just raise the engine speed a bit while watching the gauge. No need to rev it up alot, just gently raise it a bit, and see if the pressure drops into the mid 20's. I had a 240D and 2 300D's that did this very thing. After the AC is fixed, maybe run a bit of diesel purge (in the isolated manner) through the injectors to help clean them up. But make sure to check your Idle RPM to ensure it's correct before adding refrigerant.

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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D