Rick, it is best to get the gross leaks ruled out before you start looking for the small, hard to find items. If it won't hold a vacuum, then there's no point in trying to get it to retain 2 or so pounds of R-12. My local A/C guru suggests blowing in some UV dye and adding R-12 until a low side pressure is readable on a gauge (about 5-10 PSI), bypasss low pressure switch (you've already done that). Watch to see if any pressure builds on the high side. (suction side will be low or negative). Continue adding R-12 until 50-60 PSI on high side with compressor off. (read as shut off engine). See if system maintains this level of pressure and start scanning for leaks with black light. If it got cold enough scan the condensate from the evaporator drains to see if the UV is coming from the evaporator.

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Jeff Lawrence
1987 300e
1989 300e
2000 Dodge Grand Caravan SE