Somehow the sight glass at the dryer had a crack (1995 280 C) and all the refrigerant escaped so I just replaced the dryer with a new unit and added 2 cans of r134 it worked fine for about 3 days now the a/c only works at freeway speeds (very cold) but at idle and at City driving it’s not cold at all, I open the hood and I can see thru the glass the r134 is not circulating, I can see the dryer full of r134 but it looks still until I rev up the engine but then the flow stops completely for a few seconds and then it starts circulating again, any idea why is this happening?

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2000 430 ML
2005 S500
87 300 D
61 300 restoring