Fire the engine up, turn the a/c to max, roll the windows down, open the hood, and watch what the compressor is doing. If it runs for a few seconds and shuts off, and then turns back on again, it's probably low on freon. If it keeps running, and you still note that the air blowing out of the vents is warm, and then cold, and then warm again, you probably have a blockage at the orifice tube.
I'd put a set of refrigerant gages on it, and watch the low and high pressure while it's running. If you see the low pressure dipping below 25, you know you're low on freon. If you see something else, let us know.
The system is R12, so you can't just add R134a. You have to either find a place that still uses R12, score some R12, or have the system emptied and then retrofit w/ new receiver/dryer, new orifice tube, replace as many o-rings as you can get to w/ green viton versions, and then add a few ounces of Ester 100 oil and then add R134a until you have a steady 25-30 inches of suction on the low side. It'll blow ice cold.
