Larry can probably help you better, but you can purchase a leak detector which will run you a few hundred. You may want to take it to an indy if you can't get your hands on one. THe best place to check is the evaportator and the welds in the piping. Good Luck.
1957 Dodge D100
1967 VW Microbus 21 Window
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"It cannot be emphasized too strongly or too often that this great nation was founded not by religionists, but by Christians; not on religion, but on the Gospel of Jesus Christ. For that reason alone, people of other faiths have been afforded freedom of worship here." Patrick Henry 1776