Use an R-12 air conditioning recharge kit by first making sure the ambient temperature is above 55 degrees Fahrenheit, locating the low side service port, connecting the refrigerant hose, and charging the system to the specified capacity. Some recharge kits come with simple gauges to monitor system pressure, but if your kit does not, you must use a refrigerant hose with a manifold pressure gauge to avoid over or undercharging the air conditioning system.
Ambient temperature must be over 55 degrees Fahrenheit because the system used to determine appropriate pressures only works above this temperature. Most R-12 recharge kits come with a small card or chart that lists necessary high and low pressure values according to ambient temperature. If your kit does not include one of these items, aim for a low pressure reading around 30 psi, and a high pressure reading, again in psi, of about two times the current ambient air temperature in Fahrenheit. The two times ambient temperature guideline only works for older R-12 refrigerant, do not use this method when charging a vehicle that uses R-134a.
After charging your vehicle to the appropriate pressures and resealing the air conditioning system, close the hood and run the vehicle for a few minutes with the air conditioner turned to full. If the air does not blow cool at all, or if it blows cool for a few minutes before quickly becoming warm again, a leak likely exists somewhere in the system.