Cadillac Repair: A/C, 134a freon, r 12 freon

Question
I have a 1992 Cadillac STS and just added freon 134a. I had to convert the ports to accept the 134a. Now after adding 18 oz of 134a the information on dash says A/C compressor is off, very low refrigerant. Do I need to add more freon? Do I need to reset the computer somehow? If so, how do I do this? Please help. I need to drive through Vegas this evening and it's hot, hot, hot.

Answer
Did you remove all the old R-12 freon from the system ?.

Did you use a vacuum pump to remove all the atmospheric air from the A/C system ?.

Did you install 85% of the correct manufacturer's required amount of R-134a freon in the A/C system ?

NOTE: Most Cadillacs CANNOT be converted to R-134a freon due to the fact that the LOW REFRIGERANT light as well as the CHECK ENGINE light will illuminate because the LOW PRESSURE SENSOR is NOT calibrated to accept R-134a freon. You have NO CHOICE but to use the R-12 freon.