I have found the red dye to be pretty much useless. You need to use UV dye and then search with a black light. You can get an inexpensive UV dye kit that has an injector sort of system with oil and dye. This is usable on R12 and R134 systems.

Once you have charged and have the dye in the system, look at the condensation drain water under the car with a UV lamp, preferably in the dark. If there is UV dye in that water, the problem is your evaporator. Do this soon after you have charged with UV dye because once the refrigerant leaks out it won't cool well enough to make condensation drain.

If the condensation drain water is clean, then keep an eye on all connections with the UV lamp. Common leak points are the expansion valve and the compressor front seal. You have to pull the front nut on the compressor and pull off the front clutch plate to check the shaft seal.

Good luck,