We recently received an e-mail from the proud owner of a 1963 Cadillac, with installed, under dash air conditioning manufactured by Classic Auto Air. The compressor is disconnected, and the owner is curious about how much R134a is needed to re-charge the system. 
Instruction manuals are included with each Classic Auto Air system shipped, but we get this question frequently enough so we are answering it here in our Tech Talk section.
When car owners call or e-mail Classic Auto Air for technical assistance, they usually start with the make, model, and year of their car. In this case, determining the proper amount of R134a needed to re-charge the ac system isn’t really driven by make and model, but rather, by the style of air conditioning system installed on the car.
For a 1963 Cadillac with an under dash evaporator system, the recommended charge of R134a is 22 ounces. On all Classic Auto Air under dash units, 22 ounces is the recommended charge. On all the Perfect Fit evap units, 24 ounces is the recommended refrigerant charge.
It is important to remember that all A/C systems operate on a closed system. Whenever the system has been discharged for a period of time or has been open to the atmosphere as in the case with this Caddy, it is highly recommended that the drier be replaced before re-charging the ac system. Failure to replace the drier can lead to a poor performing system along with the possibility of drier failure. If the drier fails, it can contaminate the whole system. The desiccant that is contained in the drier can clog the expansion valve and evap core making repairs very expensive.
To guarantee best results, the owner of this 1963 Cadillac should take the following steps to properly repair the air conditioning system.
- Find and repair the leak or replace the faulty component (compressor, condenser, hose, etc.) that caused the system to lose its charge
- Replace the drier
- Evacuate the system
- Charge system to recommended levels