Thanks topgun, how did you localize the leak to the expansion valve?

leathermang: yes, everything else should be in good shape.
Yesterday, took the car out and the AC worked - sometimes it takes about 10 min of driving before the compressor kicks in. Once that happens it gets cold (though not as cold as when I first repaired the system last summer.
Low pressure side remains steady at 20. Haven't check HP side yet.

Definately getting blips from the vents, if I stick the tip down into the vents. The most blips come from the far passenger side vent.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT