R-12,??? What year is your truck? Even my 93, WT had 134-A when it was new. That being said, if it is a R-12 system you have, yes, it can be converted to 134-A. To do so, you have to comply flush out the system, get the adapter kit, install the fittings, pull a vacuum on it, and recharge withe 134-A. You CAN NOT, just add 134-A to your system. The freon will turn into a thick nasty mess, if you do. R-12, and 134-A, are not compatible. Also you will need to replace the refrigerant oil as well. These jobs, are not for the novice mechanic. Also, if your low on freon, then you have a leak somewhere. You will also need to fix that leak. IMO, your best bet, is to leave it alone, untill you are ready to replace everything. It's not good to open up the system, and just leave it open for a period of time. This allowed contaminates, and moisture to enter the system. To do the job rite, it takes, some AC flush, an air compressor to blow that through the system, after you open the system up, a set of AC gauges, an AC vacuum pump, and even a scale to weight the freon, plus all of the parts needed to replace what ever is not correct in the system. You will need to replace the office tube, along with what ever other parts that are leaking, and most likely the condenser core. Also, there is the issue of capturing the old freon. It is illegal to release freon into the atmosphere.
