Most of the older cars are hard to find any kind of leak....They just seep a little until the charge is weak enough to not cool. Unless you want it for winter, I would wait until next spring to recharge it.
The debate is real hot on what refrigerant to use so I won't go there. Your system was designed for R12.

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.