Is there a LOT of dye at the rear of the compressor?

When you put in dye, especially in an R12 system where the refrigerant is expensive. You need to use your black light while the system is charged and working well rather than waiting until it is empty. That way you can look at the condensation water to see if it has dye in it. If there is then it is the evaporator. Once it is empty the only way to confirm the evaporator is to tear everything apart to examine the evaporator.

If there is only a tiny trace amount of dye at the point you describe, it may not be your main leak.

Best of luck,