The first step is to find the leak and then evaluate the components to see what you're in for..
1.See if the shop can find it with an electronic (heated diode or corona discharge) or sonic leak detector.
2.Have remaining R12 reclaimed at the shop.
3.With that large a leak you may be able to hear it with a pressure test or even a vacuum test (not always reliable with a "flapper" type leak.
4.If it needs to run for a while, or you want to find it yourself, put in some UV style dye and charge with an HC refrigerant (for test purposes only; no flames (of either kind), please) so you don't have to mess with wasted R12, oil compatability, polution, and re-evacuation.
5.In a day or so look for the dye at the system leak(s).
Evaluate components and see if you want to proceed. (an evaporator leak will make you think of living with the window down).

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)