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to check for leaks, the system must be charged.
a UV light will only work if there is leak detector dye in the system . UV will not work with only freon in the system.
the best way to find a leak is to use a "sniffer".
and don't be surprised after you go through all the trouble to charge the system to find a leak and don't find one in the engine bay.
a common place on ford truck A/C systems to leak is the evaporator core in the heater box.
TJC here brings up a valid point about the leak detection dye!
OP,
you can get the small cans of freon at your local autozone with a leak detection kit if i recall correctly.