You mentioned that the low side pressure isn't right - If the low side pressure is low, it is a for sure indication that the refrigerant charge is low. Watch your sight glass and have someone turn on the AC - you should see bubbles, then clear as the refrigerant becomes a liquid on the high side of the compressor. Feel the return line to the compressor with the fan on a high setting - it should be cool to cold going into the compressor, if it feels warm there's not enough refrigerant charge.

If you have converted to R-134, be aware that now the condenser is 1/3 too small for the rest of the system. You would see a huge performance gain by going to a parallel flow condenser.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C