I had same issue, had dealer evac, and recharge system. Works perfectly, however they did find a leak at the compressor. Right now I am refilling when needed. The recharge by the dealer lasted about 6 weeks with minimal use of air. I am on week 2 with my own refilling via can from Autozone. I am hoping to get through the summer this way until I can get the compressor and drier replaced. Also, low pressure will cause the expansion valve to hiss, its a sure sign you are low on R134

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Dennis
2006 E320 CDI Black/Black Leather. Nav, HTD Seats HK stereo 32000 miles.
2000 VW Passat 1.8t 300,000 miles as of Dec 09, 2009 original owner
1997 E300d Silver 300,000 miles As of Jan 05, 2010
3rd owner SOLD