You do not indicate where you are located. There is a different answer for this question depending upon whether you live in Anchorage or Houston.

If you have a leak under the dash it could be a line, the expansion valve or the evaporator. To keep from losing refrigerant the best thing to do is take it to the local automotive a/c shop. They can put in 4 oz. or R22 and pressure it with Nitrogen. They then use soapy water to look for bubbles. This is how they do it without losing any of our precious R12.

On this car, unless you live in Anchorage or at least North of the US border, you should recharge with R12 once the leak is fixed. It now only costs about $29 per pound and will give you cold air. Some cars convert to 134 well and some don't. The 123 is definitely a car that is in the "don't" category.

Some people insist on spending bunches of money to convert to a less expensive, lower performing refrigerant, rather than spending sometimes less money on the proper refrigerant.

You have to fix the leak regardless of the refrigerant you later put in it. You will be happier with the performance of the R12.

My $0.02,