Rick, before you toss away $$$ on R-12. Have the shop pull a vacuum on your system. This is the least expensive way to start. If it doesn't hold a vacuum, you haven't contaminated the atmosphere or watched you $$ float away to the stratosphere. If it does hold vacuum, then go with a partial charge leak test.

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