The old Chrysler service manuals mention using a partial charge to check for leaks prior to doing the final "full" charge of freon. That usually was about a 1lb can of freon back then. Enough to let the compressor run OK, even without the later minimum charge sensor switch, and still check for leaks. If that leak test with the swept charge was successful, then the rest of the 3.5lbs was added.
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