You pretty much have to have an open line to get air/moisture in the system. Even when the AC is static and not being used, its 50PSI or more over ambient pressure, nothing is going to leak in. If the system leaks, pressure will drop, but still stay above ambient pressures, keeping anything from leaking in. The low pressure switch, ~27PSI for an R-134a system, will keep the system from turning on at all if the pressure is below that. If you can get the compressor to engage, even it its just a second, your above that pressure. Sure it possible to leak out all the refrigerant and pressure is reduced to ambient levels and let air/moisture leak in, but its very unlikely, it usually only happens with broken/open lines and blown seals that let all the refrigerant leak out within seconds. Usually with slow leaks from detiorated seals, the seals still keep enough pressure that its above ambient and doesn't let air/moisture in.
If you have a very slow leak, like it takes several years for the AC performance to detiorate, then tighten all your lines, that sometimes is enough to stop the leak, or at least slow it down to a lot less.
R-12 and R-134a systems have different pressure ports, so that you can't accidentally fill it with the wrong refrigerant. So if the R-134a fill tube you bought fits, then you have an R-134a system. I have a '95 and its a R-134a System. I "THINK" the last R-12 systems for all cars was phased out by '92.
The best way to service an A/C system is too put the proper pressure gauges on it and properly diagnosis the problem. If you do an improper diagnosis, which if you don't know the actual pressures in the system, you can't do a proper diagnosis, you can waste a lot of money on improper repairs or damage the A/C system, which costs a lot of money.
That being said, if you don't want to spend the $80-$120 on the A/C gauges, it does sound like your low on refrigerant. IF YOUR WILLING TO RISK IT, then yea, fill it with a can or two of refrigerant.
If you don't have gauges, which is the only way to get a close to proper servicing level of refrigerant, than the closest way to get the proper level is too: Watch the low pressure/return line (the cold one) from the firewall, as you add the R-134a watch that line near the firewall, it should get cold enough that water starts to condense on the line. When the water condenses enough that it starts to bead up/sweats, add another once, continuing charging a few seconds more, then you should have enough.
With the motor running and the A/C turned on full (in recirculate) with the fan going as fast as possible (windows open helps), hook up the charge hose and start charging the system with R-134a. You hook it up too the low pressure/return line, the fittings are different between the 2 lines, so you shouldn't be able to screw it up. Remember the low press/return line is the one from the firewall to the compressor, the high press/supply line is the one from the compressor the condenser. Make sure the can is upright, upside down then liquid refrigerant will run thru the line and can stay liquid long enough that it can hit the compressor and damage it, keep the can upright and only evaporated refrigerant/vapor will enter the lines. Have a bucket of very hot water nearby, as the charging starts to slow and the can is running out of steam, drop the can in the bucket of hot water (still hooked up to the A/C), the hot water will get the last of the refrigerant in the can to evaporate and get it into the system. Stop charging a few seconds after the return line starts to sweat fully, like I said above.
Remember the pressures in an A/C system are very high, the refrigerant in the can will move very slowly, it may take 20 minutes a can to get all the refrigerant out. Just shake the can, you can tell if any is left over. Remember, even the low pressure side (also called the suction hose) is at a high pressure, its called low pressure/suction because thats realative to the rest of the system, relative to the outside air, the low pressure/suction side is still high pressure.
The A/C has to running to charge the system, to create a suction/lower pressure on one hose. If the A/C is not running, the pressures in the system will be higher than whats in the can and no refrigerant will move out of the can. Only when the A/C system is very low or empty is the pressure low enough that the refrigerant will move out of the can with the system off, and then it will stop when a 1/3 of the can or less has entered the system because the pressure will raise enough to equal whats in the can.
If your A/C is not even coming on because the pressure is too low from the leak, still charge it just like above, even though the compressor isn't coming on. As you add refrigerant, the pressure will raise enough and the system will turn on and you can continue just like above.
Consider adding a can of 1 oz of R-134a oil, preferably PAG, the oil mixes with the refrigerant and can leak out with it. You need to have enough oil to keep the system completely lubed and lasting as long as possible. Thats the way to find leaks, is too look for the residual oil left over from the leak.
