Most Freon leaks are around the compressor and/or the hose fittings. I have found many leaks to be the charging ports.I don't think you can buy the R-12 with the red dye to detect leaks, but I believe you can buy the R 134a with the red dye. Make the conversion and put a can of R-134a and one can of the red dye in the system, and see if you can find the leak.
You may have to use a jumper wire to jump the low pressure switch on the receiver/dryer to get the comp. to come on.
I use a hair dryer to heat the can of Freon to get all the gas out. Or you can use a pail of hot water.
Bill Lewallen Lexington,Ky.